A lot of people will say Xander but I would advice against it.
I ran the calculation, Xander won't survive an unbuilt Celica :
Celica, Caring Princess gains 5 attack for having full HP [Ragnarok].
Celica, Caring Princess gains 5 speed for having full HP [Ragnarok].
Xander, Paragon Knight gains 6 defense for getting attacked by a red tome user [Distant Def 3].
Xander, Paragon Knight gains 6 resistance for getting attacked by a red tome user [Distant Def 3].
Celica, Caring Princess attacks. 28 damage dealt. Xander, Paragon Knight HP: 44 → 16
Xander, Paragon Knight counter-attacks, ignoring distance [Siegfried]. 26 damage dealt. Celica, Caring Princess HP: 39 → 13
Celica, Caring Princess makes a follow-up attack. 28 damage dealt. Xander, Paragon Knight HP: 16 → 0
Celica, Caring Princess takes damage after combat [Ragnarok]. 5 damage dealt. Celica, Caring Princess HP: 13 → 8
So unless you can provide him a Fortify buff he will die. Even if he has a Hone buff, he still dies to Celica. It's worth on him, if you want to bait Bow users but BLyn had sacaes blessing.
He will need a dedicated horse team to use it
If you have HJakob, go with him.