Some units don't shine unless they grind. It's a sad truth, but grinding is oftentimes worth it.
Unfortunately, Sully will almost always have average stats, but she's a pretty good unit regardless. She usually ends up being one of my best Paladins due to having access to Swordfaire, Luna, Lancebreaker, Aegis, and other useful skills. I tend to recommend these four, however, since they're worth having on a unit that primarily wields swords. She also makes a viable pair-up support unit with access to Deliverer, Defender, and Dual Guard+. While Sully may not always be the best fighter, she's versatile in overall usage.
Cherche bears the curse of being a decent unit with awful reclass options. The only skill worth picking up if you plan on using a front-lines Cherche is Renewal from the War Monk line, which helps to mitigate her atrocious resistance. That way, if she's hit with a spell, she can replenish some HP on the next turn. You could also opt for Dual Support+ and Deliverer from the Valkyrie and Griffon Rider classes respectively if you want her to be a good pair-up escort.
I don't tend to use Maribelle, so I don't have any information on her.