I recently started the game, and while I've found Pia starts off as a beast (70 attack/matk!), Raven's nowhere near that strength. Is there any way to increase a partner's stats besides leveling? Thanks in advance!
Well Pia SHOULD have more weapon attack since she uses great sword (a slow but heavy hitting weapon). Raven can probably do more in a meleeing contest since she attacks faster; great sword's strength is spamming rune abilites, which Pia don't have. And yes, check their equipment too since I believe Pia starts with a higher tier great sword than Raven's short sword. You can give them better weapons (and one piece of accessory) but giving it to them like you would a gift. Level and powering up items do not affect their ATK and MAG ATK, only their STR and INT is affected but these two stats are bascially ATK and MAG ATK as well.
Also, if you plan on using Raven you will find that her strong points are her magic; in fact you'd wish she was a pure mage and never ever use normal attacks. I recommend giving her a Curse Charm (which will use her ATK and STR for determining Magic damage unless her MAG ATK and INT are higher) and maxing her ATK. Once you get the Courage Badge you can go all out MAG ATK. and INT.
After you've completed Raven's quest line, she will have the strongest fire magics in the game and fire magic is easily the best kind of magic due to being able to abuse the item oil.