The Justice Arcana
This Social Link is tied to the really shy girl you meet when you join the Student Council, Chihiro. You can find Chihiro standing just outside of your classroom nearby the Student Council room on Tuesdays, Thursdays and Saturdays (as seen in the picture below). Since she is extremely shy around boys you will have to talk to her on 3 different occasions and give her responses that don't scare her before she'll do anything with you.
After talking to her on 3 different occasions and getting her to walk home with you you'll unlock the Justice Arcana.
While raising a Social Link you'll be given story cutscenes where you'll have to answer questions. The answers to each of these questions is different depending on the person and the situation. What you usually need to try and do is pick an answer that reflects their personality. Here's a cheat sheet for each Social Link rank of the Justice Arcana and what to say.
Rank 1: Automatically earned
Rank 2: Don't worry about it - I read manga - Not really
Rank 3: How rude - I agree
Rank 4: Anything - Speak to her calmly
Rank 5: How was it? - It's just a coincidence - Don't be so hard on yourself - Hold her hand
Rank 6: Is it good - What do you mean?
Rank 7: You're not to blame - We have to do something
Rank 8: Anything - Talk to the teacher
Rank 9: Don't worry, she'll help us - (Wait and see what happens) - Anything - What's wrong
Rank 10: Automatic
Upon completion of this Social Link you'll be able to fuse Melchizedek through regular or triangular fusion.