I haven't played the game for that much. but based from what I experienced (and also personal preference) in terms of combat prowess, I think Sandra is best. 1. she's a dual wielder idk if that attributes to dealing more damage but if it does then she already has the upper hand compared to other girls. 2. based from what I've seen: there are more elemental daggers compared to other weapons. 3. her skill gained via laboratory is also pretty op (the 3 hit atk) 4. She can easily farm gold for the team with the use of robbery. 5. her agility is pretty whack. she can have more turns if you pump her with a lot of agi items. 6. She's easier to use compared to Elly(Elly can be pretty op but you need a lot of TPs to do so. crit buff + 5 hit attack). She has a bit of downsides though 1. She's defensively weak to spells and physical attacks, but this can be remedied by feeding her defense and resist potions. 2. She lacks AoE which makes killing mobs longer. but it's a 4 man team anyway you can always just have Chiyo , Kayelinth and/or Mira(for undeads) in the team to deal with this problem. 3. giving her most of the attack/defense potions would mean less for Ryen. That could be bad for Ryen exclusive missions.
on the looks department. no issues there. all the girls are pretty. so nothing more to add.