Takeba Yukari
Yukari and Mitsuru start out at odds with each other. Yukari feels that Mitsuru is a cold and untrustworthy woman. She wonders why Mitsuru "always has to be like that" when she requests help because Mitsuru has a definitive way of asking (it seems more like telling). Yukari joined SEES to find out more about her missing father, she believes that Kirijo group is hiding information. She later confronts Mitsuru about it and finds that the only member of the group that survived the explosion was her father. This leads the group to Yakushima. There Yukari learns that her father had a hand to play in creating shadow and tartarus. Mitsuru is genuinely surprised that the scientist on the video that they watch is named "Takeba". This leads me to believe that although she covered up (or didn't tell) things about the incident she didn't know that Yukari's father had been present. I feel that she certainly didn't meant to hurt Yukari or the others by not revealing all of the information. I think she meant to tell them exactly what they needed to fight and left it at that.
I would say that Yukari is Mitsuru's first female friend (I wanted to say first friend, but what about Akihiko and Shinjiro). Despite initially distrusting Mitsuru, Yukari understands what Mitsuru is going through when her father passes away. Yukari put aside her own feelings of anger and distrust to assist Mitsuru in her time of need. After the loss of her father, Mitsuru fell into depression and lost her will to fight. Yukari had several reasons to hate the Kirijo group as they caused her much grief, but having been on that side of the fence she knew that what Mitsuru needed wasn't more scorn or distrust. It was hope and friendship. Yukari offered that. I believe that it was the first time Mitsuru cried in front of anyone and her vulnerability made her much more human to Yukari. It's in part what really made their friendship. Grief over their lost fathers was able to bind them and push them forward into friendship sharing first the bad and later being able to celebrate the good. It is through the resolve and friendship with Yukari that she gains her new persona, Artemisia.
In Persona 3 FES The Answer, Yukari's heartache over the loss of protagonist leads her to wanting to find the answer and save him. Mitsuru keeps her own feelings locked away, never fully stating who she actually agrees with. Instead, she stands with Yukari because Yukari stood by her. I find this a very charming and giving situation in which Mitsuru puts her own thoughts and feeling on what happened with protagonist away and sides with Yukari. Simply because she felt that Yukari needed extra support. I think this is the mark and measure of a good friend. Someone who might disagree with you but stands by you because they value you and the things that you've done for them. Mitsuru is not without appreciation for her friends and when called upon will defend them with fervor! For me this goes back to her Empress Arcana in which she can be very nurturing and very cold.