Well having a Lamia on your party might be interesting, I mean Makoto's party are one Weak Mage/Assassin and one Strong Mage/But long time to cast. Having a physical fighter could be useful to cover their weakness. The problem is, if she needs to turn into Lamia to fight, then MC will never be able to party again, unless with his merchant otherworlder friend, since everyone else will kill her for being a Lamia.