Kirijo Group
Mitsuru's devotion to her father is what keeps her fighting. She tries desperately not to bother him and apologizes for stopping in. She mentions that she is happy to see him in good health. Although she loves him, she has a difficult time talking to him (as I am fairly certain most teenage girls do). This causes tension in their relationship. Mr. Kirijo tells Mitsuru that she needs to learn to trust others and tells her to bring the group to him.
I get the idea that Mitsuru had a very strict upbringing. I also believe that she trusts her father with unending faith. Her issue is that she's trying so desperately to handle everything that she doesn't want to add more to his burden. She knows that he is busy, and this is why she tries to accomplish things by herself. Her desire to fight alone, however, prompts well-intentioned scolding from her father. He insists that if she wants information all she has to do is ask. I feel that Mitsuru wants to help her father in a way that doesn't cause him extra trouble. She would be content to shoulder the burden all by herself. Her father doesn't want this as he constantly reminds her that two in harmony surpass one in perfection. I can also understand her awkwardness about talking to her father, she's an only child in a rich family. I doubt she got to speak to her father very much after shadows and tartarus were created - even though she wanted too. Mitsuru is the type of person who has to take care of others. She needs to be able to take the majority of problems and solve them. She wants nothing more than to look are her father without causing him extra grief. I think that she feels she will be troublesome because she knows that he has other duties that he still has to attend too. I also believe that her behavior directly stems from her father's. She has seen him fight and handle Kirijo group by himself, rather that he does everything he needs to by himself. I think that she adopted her stubborn streak and independence from her father. I don't know that she could be any other way, considering how much she loves him - I imagine that he is her hero.
Without persona, her father is unable to fight. Cleaning up the mess of Kirijo then falls to her. With this though, it stands to reason that she is used to fighting on her own. She's used to doing what she needs too, and this is probably why she rarely opens up. She's accustomed to doing things a specific way. Look after father, clean up dark hour without troubling anyone else. Her best intentions are usually left unsaid and the silence is misconstrued.
Mitsuru's grandfather is responsible for the creation of shadows and Tartarus. This is something she keeps to herself until confronted. Yukari yells at her stating, "Is that all we've been doing?! Cleaning up someone else's mess?!" While this is true, it hurts Mitsuru deeply. She hadn't meant to cause trouble, Mitsuru only meant to keep irrelevant issues from them. Yes, she could have told the group, but what good would it have done? Mitsuru gave them all a choice to join and they did so of their own free will. I have never felt as though she mislead them. In fact, I believe it would be extremely hard for Mitsuru to open up to others.
Mitsuru gained the ability to use her persona versus being born with it (like the rest of SEES). She came into her ability through the experimentation of the Kirijo group. She is finally able to summon her persona to protect her father when they are attacked by shadows in Tartarus.
During a vacation to Yakushima, Mitsuru meets with her father. He gives SEES the information that tartarus and shadows were created as a byproduct of his father's insanity. It is through this that we learn that Yukari's father also had a hand in shadows and tartarus. Mitsuru didn't know that Yukari's father was present but Mr. Kirijo did. He regrets that his daughter and her friends are the ones that have to clean up his father's mess.
SEES is then lead to believe that defeating the twelve major arcana shadows will stop Dark Hour, shadows, and Tartarus. With their new goal in mind they march every forward facing challenging shadow bosses every full moon. Later, Ikutsuki betrays the group and kills Mr. Kirijo. Along with Mitsuru we learn that Ikutsuki has been orchestrating the ordeals and that he shared her grandfather's insanity.