Best red unit to make it into tier 20

Well, the most "common" red units are probably units like Ayra, Mia, Soleil, A!Tiki, Zelgius, Hana(?)... I guess Karla as well, maybe?

If you're wanting to make it into Tier 20, then you might want to use some units that are easier to merge, unless you're going to be spending all of your orbs trying to get 5* exclusive copies.

Zelgius if you have unlimited orbs; otherwise, Draug because of BST and mergeability :/
If you don't like armours, pick an infantry or flier sword with at least 160 BST such as Soleil

EDIT: checked and found that Soleil is the only 160+ BST non-5* exclusive sword other than Draug who also has +10 potential. Maybe the 155-159 is more viable for scoring than I initially thought? So Eirika, Marth, Hana, Ayra, the whole company is game. A.Tiki is also in this category and performs excellently at the top levels of arena

It really depends on what unit you have a lot of copies of to merge. Tier 20 usually requires a decently merged unit. That's the most important. After that, units with higher BST are better. So armors are the highest scoring, and usually dragons and more recently released units all have boosted BST too. Skills are also important - the higher the SP cost of the skills your units have equipped the better.

Infantry or better armored. The way scoring works Cavalry are nearly locked out because of their lower BST values... I can't think of an easy way for IS to fix arena, and I doubt they want to since it caters to super merged whales.

Draug will give you the best bang for your buck, seeing as unless you have an inordinate amount of money lying around, the only real reliable choices for T20 are units in the 3/4 star pool. Soleil, A!Tiki, Ares and Eliwood score fairly decently as well, seeing as Soleil is the only 160-164 BST red at an available rarity and the other 3 can hit the same scoring bin if you give them DC (sidenote, for this to happen with Eliwood, you need to avoid banes in HP and Speed). Those 5 would be your best bets.

Edit: Theres a handful more units that fit into the same category Tiki, Eliwood and Ares do, I just choose some of my favorites from the category.

After playing since launch, Soleil is actually an incredible unit.

Merging is essential to make it into tier 20, so plan long term even if it’s not in the near future. You will likely regret it if you don’t. Like me, I never even considered merging 5 star units considering I am F2P. Now I have my main team with a +4, +5, and +6 merge. They could be a bit higher if I didn’t waste copies of them on unnecessary skill inherits.

Best advice is to 5 Star the askr trio and build them, a guaranteed weekly bonus unit is a must. Might as well build them to be reliable and increase your score.

Also no one mentioned Roy as a red. He’s my newest addition to my core team. At first I was skeptical but now I’m glad I committed to him.