L!Ryoma and Fjorm

Since they boost Defense and Speed respectively I'd narrow the choices to units that have serviceable stats in those 2.

2nd you'd want a unit that can deal with some of the stronger Blues like Hardin, perhaps a green, especially if you are keen on having the color triangle covered.

Depending on who the 3rd unit is that would help you choose a 4th.

For instance Merric would be a good choice stats-wise (Green Mage with serviceable Speed, and good Defense), however using his prf weapon doesn't synergize well with them at all so he'd need another mage or a staff ally.

If you want to play +spd Fae she would love the extra spd and def. (However, I tend to pair her with L.Lucina also for extra spd but also for distant guard.) Along similar ideas, Camilla or Libra or W.Lissa, who can really appreciate Ryoma's guidance.

Saizo likes spd and def.

Nino likes spd and def in either variation of her. She tanks DC counterattacks better this way. Same with a lot of mages, actually.

Zelgius likes extra spd and def.

Felicia, Kaze, most daggers with higher res than def, and some builds of Innes like extra spd and def.

Guard bow Rebecca (and possibly other builds of her) like extra spd and def.

Some mid-to high-spd healers like extra spd and def. Sakura and Clarine are the two first examples to come to mind.

Barst and spd-invested Bartre like extra spd and def.