Team suggestions?

this question is to vague as it is currently, What are you hoping for the team to do? is any unit an option? what roles are you hoping to fill --roles not as in color but as in use--?

When building a team, I typically try to have each unit fulfill a role.

For example, some of my teams have: a unit that's main job is to kill/rush, a unit that can tank physically, a unit that can tank magically, and a support unit.

Also, I try to balance out the colors by having 1 red, blue, green, and the last unit can be either a repeat color or a colorless.

Since it seems your Leif is your rushing character, and your Kliff is your mixed tank; you might need another off-tank to help Kliff against units he won't be able to tank and a support unit that can buff/debuff.

B!ike with double drive is a decent support tank because he can go into danger without worrying due to urvan's affect.

bridal Sanaki has res/def link which is good on Kliff and can tank mages and blues.

Guunthra or S!sharena can help with ploying and debuffing.

L!robin takumi, Saizo, or azama can be a substitute tank if leif or Kliff can't tank. HS!Xander can both tank and dance.

Lets give you a few options here, Firstly lets recognize what you have;
Kliff;
Good mixed bulk gotten even better when his tome, making him pretty refined into a tank/mixed phase role.
Leif:
Leif is an okay unit with no gimmick and there are many, many other swords that outshine him. All of his stats are mediocre, but can get better with right support.

Next, what you are missing;
Support unit;
A support unit is a must, Leif is outclassed by many in his genre, however, if we can apply some good buffs to him, he could perform really well. A healing unit would also do well to help keep Kliff alive as he tanks. A refresher could also help both units as are going to have a little trouble confirming the kill with their low atk.
Nuke unit;
As just stated, both units are going to have trouble confirming a kill on some of the more tanky units, including a good nuke role will ensure the death of many harder to kill units. With both Kliff and Leif being mixed phase, a good player phase unit will elevate the team.
DC unit:
Though Kliff is a good tank, he suffers greatly from not being able to make the most of his def. Running a strictly enemy phase unit will patch up the holes Kliff naturally has.
Anti-team unit;
Sometimes those dang armor/flier/horse embla team are tricky to deal with depending on how your team is built. If you go for a more tanky team than you suffer on attack and armors are a problem, go for mostly offense and prepare for rein/lyn to tear through the low defenses.

Now lets theorize some units.

First up is armors.
A great nuke unit is H!Jakob, he can run a quad build that will kill almost any unit in the game, he can also change into a fireswept or guard bow build also, making him extremely useful.
Pair with a F!Robin(M) --or Grima, for the sake of this paragraph--, Grima as a great unit that can take on almost any opponent. He fills the role of DC unit perfectly with his DC breath, and tanks very well thanks to coming with vengeful fighter. Due to these two skills being on him already, he is extremely cost efficient to build, and can fill a support role if needed by filling in his C and Seal with buff skills. slap on fury or A skill of choice and viola. With his high HP it is also a good option to run Ardent Sacrifice for some light healing. Do not mix Ardent Sacrifice and Fury however.
His weakness is to Falchions, but Kliff should be able to check them.
H!Dorcas is a budget replacement for Grima.

Both of these units can be fully built with current banners also.

Next, H!Mia/Maribelle/B!Veronica and Nino/S!Sherena/WT!Olwen/Julia
Offensive healers are great colorless options, as they are great buffer/debuffers, attackers, and healers.
Elise isn't listed because she doesn't come with a support staff, Maribelles' Trilemma will almost guarantee your units correctly paired matches, H!Mia can completely shut down the threat of armors using specials --such as Black Luna, or a vantage LA!Hector with bonfire--, and B!Veronica staff straight up buffs you and debuffs the enemy.

Nino, S!Sherena, WT!Olwen, and Julia are all green tomes who excel at attacking.
All of these units can run blade builds, to monstrous effect, the best users of the blade tome however are Nino, S!Sherena, and WT!Olwen, as they all have high speed. The blades are going to greatly appreciate the buff from B!Veronica.
Julia comes into her own because of her effectiveness towards dragons, making her a great check for many of the best units in the game.
S!Sherena's tome Muninn's Egg is equipped with a Chill Atk/Res 2 effect built in, making her extremely useful, though her debuff in atk will make it harder to pull off Kliffs effect.
WT!Olwen will be even better with paired with one of the horse healers, as she than has access to horse buffs.

Myrrh/Beruka and H!Mia
Myrrh and Beruka both fill the same genre, tank fliers, though Myrrhe does a much better job thanks to her weapon preventing follow-ups, Beruka is easier to invest in with merges and has gotten a buff with the release of Hack-O-Lantern.
Being fliers these units can run Guidance seal, making them great for team mobility. Both units are also extremely capable of tanking almost any foe on enemy phase. Beruka can even get to a point where arrows don't matter anymore because of her def. Both units are also amazing users of DC.
Myrrh is also able to change her breath to one of the more supportive breaths, also able to buff your units. Or she can run Lighting Breath to free her A slot.
H!Mia for the same reasons as before, but also added on the fact that she is a flier and able to give flier buffs to either Myrrh or Beruka.

Honorable mentions:
PA!Azura;
shes a refresher who easily hits 50/40 atk/res, her unique weapon also gives a visible buff of +3 to every stat when using dance.

Macaiah;
Her unique tome gives her unparalleled use, as it increases her tank and also gives her effectiveness against armor and horses. Her unique command Sacrifice is also the best healing option in the game, healing exactly enough while also having the effect of Harsh Command.
Since you seem set on only running a green and colorless, Micaiah and her pairing have been left out.

Marth/M!Corrin
Both of these boys are combat buff experts, able to buff every stat. Marth is more useful however thanks to his Falchion being effective against dragons and having the effect of Renewal 3. With the Renewal 3 effect of his weapon, he can also run Ardent Sacrifice to further his use to the team.
He has been left out for the same reason as Micaiah.

If you have any questions, feel free to ask.

Units like Sanaki, WF Hinoka, M.Corrin, and Marth who are good at giving drive buffs can give him a nice boost. Corrin doesn't even need to be ally supported to him, just use drive atk and the whole team gets it.

Infantry flash would help him a lot if you're not willing to switch to flashing blade. I do suggest a cooldown accelerator with a 3CD special.

If you use Kliff offensively, a dancer could be good. Otherwise, you probably want someone who tanks well at melee range, especially against manaketes. Beruka and Hawkeye are strong green candidates (Beruka would need a fortress def/res though) while Roy is very strong as a red.

I got a +Atk -Res Kliff so I went and killed the Ishtar that pitied me while I was trying to get him so I could boost his attack further in battle since Swift Sparrow won't effect his tome, and even with a +Atk his Tome can still get its effect running very easily, so if his tome's effect activates he's getting +9 Atk/Spd during battle increasing it to a solid 56/46, I also run him with Ophelia who runs her default Chill Res so with that and his chill Spd he effectively has 63/53 if both hits the same target (which would most likily only come around to the final enemy but hey it works) that is if everything activates

Still need a C/Seal skill, nb4 Flashing blade is already being used on someone else that needs it more