If you feel like you needed a blue unit, Sharena or female Corrin are good picks, I think. Or any of the three starters, since they show up as bonus units in the arena every so often. Sakura might be decent for the Tempest Trials, I guess? Though I don't think she benefits a huge amount from promoting. Olivia is fine as is too, since you're only really using her for dancing. Peri is meh, there are much better cavalry units out there.
Of the others you haven't trained up yet, Reinhardt is the obvious pick. Kagero is alright too, though as fewer infantry units show up these days she's less useful. Nino is good, but you need to build a team around her (surround her with Hone/Fortify/Rally skills). Marth is not bad; you could try and get him trained up now before the Hone Falchion skills start arriving.
I don't care for the others much, personally.
