Best: Farina, if Karla is possible, then so can she (you need to beat the story first), because I can't think of anyone else worth it. Pelleas maybe, for being only dark user (other than Lehran which is a big pain to get, 1 chapter only, can't get Balberith/other blessed himself, etc) and having a good personality, yet puppeteered easily by of all people vile monsters, Izuka. How he could trust that "touched in the head" crackpot is beyond me and makes me not like him for best. Honorable mention to Lyon for summoning, two S ranks, Naglfar and Hammerne, but meh-stats and puppeteered too.
Worst: Orson, not only is he a traitor with bad stats, he's a traitor that offers nothing in his inventory I wouldn't have enough off - at least everyone else but Glen (who's cool and average) offers something unique or costly. Dishonorable mention to Lehran for reasons above.