While Mitsuru is a character that is introduced at the beginning of the game, she is unfortunately not able to be utilized in battle until later, so it's very possible to miss out on how she works in battle quite easily! Which is incredibly unfortunate since she is a great addition to the main party.
Mitsuru uses a rapier in battle, which while not the most powerful weapon, is very quick. That's not where her true strength lies though! The best way to utilize Mitsuru in battle is with her powerful ice magic. Mitsuru's magic stat is the highest in the game, making her a great magic user. She is also the only character to learn Mind Charge, which makes her powerful magic even more devastating. She also learns some nice healing magic, which while not as powerful as Yukairi's, is still a wonderful addition to her arsenal.
Because of her weak strength stat, she is at her best in a party with a character like Yukari, whom augments healing skills and also relies more of magic and not strength. They compliment each other very great in battle--I'd definitely recommend using them both in a party to explore each's true potential.
Mitsuru will stop learning skills from her main Persona, Penthesilea, at level 55. Until you reach a point in the story where her Persona evolves into Artemisia, she will not learn any skills, so be very careful with her level! Below are two charts mapping her abilities from both of her Persona's.
- Bufu, Auto, Deals light Ice damage / Freezes one foe (10% chance)
- Mabufu, Auto, Deals light Ice damage / Freezes all foes (8% chance)
- Dia, Auto, Recovers a small amount of HP (1)
- Marin Karin, Auto, Charms 1 foe (25% chance)
- Bufula, Lv. 21, Deals medium Ice damage / Freezes one foe (10% chance)
- Diarama, Lv. 25, Recovers a moderate amount of HP (1)
- Spirit Drain, Lv. 27, Drains 20 SP from one foe
- Tentrafoo, Lv. 32, Makes all foes Panic (25% chance)
- Mabufula, Lv. 42, Deals medium Ice damage / Freezes all foes (8% chance)
- Ice Boost, Lv. 45, Strengthens Ice attacks by 25%
- Mind Charge, Lv. 50, Doubles magic damage dealt next turn (2.5x damage)
- Bufudyne, Lv. 55, Deals heavy Ice damage / Freezes one foe. (10% chance)
- Diarahan, Lv. 58, Recovers all HP (1)
- Ice Break, Lv. 61, Reduces 1 foe's Ice resistance to average
- Mabufudyne, Lv. 71, Deals heavy Ice damage / Freezes all foes (8% chance)
- Ice Amp, Lv. 76, Greatly strengthens Ice attacks by 50%
In conclusion, it is ultimately up to you whether to use her in battle or not, but I personally have a hard time not adding her to my main party. Her magic is incredibly powerful, and is especially useful for back-up healing if Yukari or the Protagonist isn't around to heal. I definitely recommend abusing Mind Charge.