A Who and Why: Grail Summoning! (Minimized)

Now, this list may be very long. I recommend you search for the units you like/want to see most in a short synopsis. This aims to be a baseline for all the grail units that you may or may not want. I'll be going in order of games (IE the Origin sort in the grail list) so use that if you wish. Do keep in mind, you may merge them if you love them, there is no reason not to.

Groom Marth: A strong support unit and overall a grand unit to have. Good stats, and great to have in any gametype.
Do: Great Support unit and Ok offensive axe, grand unit to have.
Don't: You're looking for something other than a support unit. He's good, but not great on offense.

Michalis: Strong, tough, but overall underwhelming. If you wish to make a tank. Other such units like Cherche in the generic pool can make a good flier tank and she is by far easier to attain.
Do: You want a flier tank or want a good bruiser
Don't: If you are willing to merge a Cherche and or his prf being mediocre.

Camus: One of the better units. He's by far one of the better lance cavalry and a great unit for PVE.
Do:If you want a good PVE Cavalry unit or Grani's Shield fodder, or want a DC lance cavalry unit that is one of the few.
Don't: If you don't want to work heavily on cav emblem, or don't want to build lance cavs at all.

Clarisse: An interesting but weak unit given the games current meta. She is great for PVE however since she can be a mass debuffer (a weaker Jaffar if you will). She's however outclassed
Do: You want a good utility PVE only unit.
Don't: You don't use debuffers, or don't want to have a PVE only unit that only works for select encounters at max efficiency

Legion: One of the better infantry axe units ingame. Fast, Strong and overall difficult to KO when built. He's a threat, and a solid unit for general and all around use.
Do: You want a more mixed bulk axe unit or a crazed berserker.
Don't: You want to build a Linus, or simply don't need an axe infantry unit.

Navarre: Overall strong, but underwhelming when other rivals such as Athena can do much better than him, and have IV's attached. He's fast, but he doesn't hit very hard.
Do: You want a fast, Firesweep or Slaying Edge user.
Don't: You would rather use a easier to merge Soleil or Athena.

Tobin: A relatively weak and underhwhelming unit, but usable nonetheless. He's mediocre but strong/usable with enough inheritance.
Do: You want a Arena assault armor check or you seriously love him.
Don't: if you have any competent sword users or are not willing to waste many resources into a single sword user who may still underperform.

Clive: Same as Tobin. His stats and setup are relatively inferior to Camus, and his other cav lancer piers are much faster and more powerful.
Do: You wish to build a arena assault red armor check or you really, just really like him
Don't: You have any of the many comptetent lance users.

Berkut: A mixed bag and surprisingly potent mixed tank due to his lance. He can become a strong dragon killer or a potent Distant Counter user.
Do: If you want a strong dragon counter, or are willing to build a ranged DC mage counter.
Don't: You have any of the other lance units and are content with their results nor don't feel like investing into Berkut.

Arden: A very potent unit. Minmaxed to hell and his followup-ring more than makes up for it as a pseudo Bold/Venge Fighter
Do: You wish for a potent sword armor, a bonfire cannon, a tank, or a generally strong unit.
Don't: You already have a merged red armor, or if you dislike armors in general, especially his crippling res.

Arvis:A great utility unit that can hold his own. He's a healing battery, a debuffer and a mage tank.
Do: You want high utility with self sustain, or want one of the better red mages. That Hair though.
Don't: You barely use red mages or if you simply have one that already fits that position (Katarina)

Ethlyn:A great healer that's surprisingly bulky given her base kit.
Do: You like healers and are happy with her good bulk and viability for a multitude of builds.
Don't: Merge into a 4 star if you don't care for the extra stats or, don't if you have any other highly merge healers.

Jamke: A relatively strong archer, but sadly not as potent as many of the others archers. Where as Jamke requires more investment.
Do: You want a bulky archer, or another brave bow user/slaying bow user.
Don't: You are willing to wait and make a better Leon given the better availability and IV's.

Julius: One of the best GHB units for player use. Able to tank any if not all mages given the right setup, able to even tank Reinhardt.
Do: You want the strongest mage check (arguably) in the entire game.
Don't: You don't use red mages often, or don't feel that you need more mage checks.

Finn: A unit that is relatively OK, but not as strong as his piers. He is much stronger and can hold his own with multiple builds.
Do: You want a strong Lance cavalry unit, versatile and very effective.
Don't: You are planning on using Camus often, or if you have other competent lance cavalry that you have already built.

Saias: Another potent mage, this time blue, and akin to his father, is as every bit as potent, but suffers due to a lack of sustain
Do: You want one of the better blue mage utilities in the game, or want a extremely competent mage tank.
Don't: You don't use blue mages other than Reinhardt often, an don't use or require more utility/mage tanks

Zephiel: One of the better sword tanks ingame. Capable of taking a multitude of attacks and able to simply stand there...menacingly
Do: You want a extremely durable tank or a menacing wall.
Don't: You barely use armors, or dislike their lack of mobility and or prefer more offensive based units.

Narcian:A unit that came at the start of the game and has fallen behind regarding usability. Outdated but usable.
Do : You want a mixed bulk flier tank, or a unit that can do low damage as to support your pvp centerpiece.
Don't: Unit that can't kill something without a fair amount of work put into him. Similar to Tobin and Clive in this respect, but with better bulk distribution.

LA!Eliwood: A great unit to merge. 175 BST and a strong blue lance armor. Great for any form of arena and or PVE
Do: You wish for one of the better arena core units available
Don't: You already have a strong armor core unit, or if you dislike armors for a multitude of reasons.

Canas: A relatively mediocre but still usable red mage. A strong magic check. Simply a stronger neutral Raigh.
Do: You wish to have a magic check or if you want to make a strong owltome mage.
Don't: You have a raigh, or any competent red mage that can fulfill his same position.

Lloyd: A fast, but soft hitting sword unit. He's usable, and relatively potent, though objectively worse than other sword units.
Do's: You want a sword res tank or you want a fast sword
Dont's:Not a all around great duelist for general combat, otherwise build a Fir. Also that art.

Linus:A strong axe user, if not the strongest infantry axe user. Similar to Raven but bulkier, and slightly stronger.
Do's:Want a strong, bulky axe infantry and or you want one of the best axe units ingame
Dont's:Don't use axe infantry or Already have a built raven and don't wish to build a bulkier version.

Ursula:A powerful but slightly underwhelming blue tome comparatively to Reinhardt. Jack of all trades.
Do's:You want a jack of all trades that can serve many potential situations.
Dont's:You have a built Reinhardt or You have any other strong blue tomes that fit each and every possible build she could inhabit.

Lyon:A unit with good stats but sadly lackluster prf weapon unless you chose to use him with it and build him around it.
Do's:You want a strong red mage with a solid niche and you like the prf weapon
Dont's:You dislike the prf weapon and You dislike his niche or have a better red mage that is better in most any way.

Valter:A extremely versatile flier unit that can rival some of the best fliers, that can be built in a myriad of cheap ways.
Do's:Versatile Lance Flier that can fit a Myriad of cheap and effective options
Dont's:Requires some work to get off started or you dislike his prf or don't wish to invest too much.

Joshua:One of the better sword units to come from TT's. Strong, potent, and a jack of all trades type.
Do's:Strong red sword with a great niche and well rounded stats for a jack of all trades.
Dont's:Dislike his Prf and you don't want to invest alot into his kit which is mostly outdated at this point

Marisa:A strong but sadly underwhelming sword unit. potent, fast, but and almost identical to Athena save for bulk.
Do's:Surprising bulk and speed make her strong and great for support and combat
Dont's:Have a built athena or speedy sword unit. Or you don't want to built a support sword unit.

BK:the Black Knight is by far one of the best uses for your grails if you are unsure. A broken special and a great stat line
Do's:If you want a potent red armor that is one of if not the strongest in the game.
Dont's:If you have Zelgius with good IV's, or don't like armors.

Oliver:A strong but not super strong blue mage check. Strong, but not too strong. Needs investment to attain that strength.
Do's:Strong mage check that serves his purpose perfectly.
Dont's: You don't want to waste on a mage check that is objectively worse than Saias, who also brings utility into the mix.

F!Robin: A sadly mediocre stat role for the tactician. She can be many things. But most of her roles can be attained by other units .
Do's: You wish to have her be a jack of all trades with multiple sets as to have a good setup.
Dont's: Don't with to invest heavily into a unit when you have one of the many green tomes that can perform whatever she does but more specialized.

Walhart:Originally mediocre, but with the inclusion of merges, a supremely strong axe cavalry, rivals Fred-boi due to prf.
Do's: If you want a stronger Frederick in the long run who is capable of large amounts of damage.
Dont's: Dislike his prf and or already have a well built axe cav(frederick)

M!Marth:To finalize the amount of merges would be the optimal situation. Has Lucina's stats with no IV's.
Do's:To finish the merges you need to attain the +10 M!Marth and to finalize a free Falchion user.
Dont's: You wish to wait to see if IS gives us more copies, and or you have a better built Falchion user eslewhere.

Gerome: A stronger, bulkier version of Michalis without a prf.
Much more effective due to more stats in place.
Do's: Strong green axe tank or brave axe user.
Dont's: You have a strong green axe tank or you already have a cherche built up.

NY!Corrin: A great utility unit that gives his teammates great buffs.Generally outclassed by other bulky archers.
Do's: Great utility and you want an archer that can do good damage and help your allies
Dont's: You have a hard hitting archer that you like, or you dont need utility units.

F!Takumi: A great unit with a more than exceptional stat spread and a good but not great prf. Won't be winning awards.
Do's: Good stats, versatile, meme strats , and Die,Die,Die
Dont's: You have potent archers or generally prefer brave bow users and don't want to build around a relatively gimmicky bow

Garon: Strong, mixed bulk and potent. All th good makings of a dragon. Negates effectiveness but niche weapon 90% of the time
Do's: Good dragon to build if you have the fodder and or need one.
Dont's: Already have a built A!Tiki who performs similarly to him in all aspects.

Xander: A strong DC sword cavalry that is a staple of many PVE guides as well as a potent unit to have for both PVE and arena.
Do's: Want a bulky sword cav tank that can be one of the better units to have.
Dont's: You have better sword cavalry built. With Ares, amongst other being great choices.

M!Kana:A strong but relatively weak unit. Units such as Nowi are easier to attain and merge and have more potential given IV's
Do's:Strong blue dragon that can rival Nowi and can do what most other dragons do with the stapple build.
Dont's:You have a Nowi or any other blue dragon that is already on the way to being merged.