Michaelangelo Tyler "Mike" Montgomery is Aria Montgomery's younger brother. He is currently suffering from the results of his parents' break-up, along with Aria, which involves several violent episodes. He is also sneaking around his parents, keeping secrets that Aria is aware of in general, but not privvy to.
Mike does not have any known romantic relationships in the television series.
Season 1
When Ella discovers that Byron has cheated, Mike picks up on the fact that she begins acting strangely. He asks Aria if she notices it too, but Aria lies to him. He is very upset when everything changes, and his parents leave the house.
To Aria's dismay, Mike starts hanging out with Noel after Aria has broken it off with Noel. Mike innocently reports that Noel is planning to out Mr. Fitz's relationship with a student -- at the dinner table in front of a horrified Aria and her father, who reacts by scolding his son for making unsupported allegations that could ruin a man's career.
In "Someone to Watch Over Me," when Byron finds out that Aria has a secret relationship with a boy, he pries in Aria's room, looking for clues. He almost discovers the truth, but Ella stops him in an argument, overheard by Mike. When Aria returns home, Mike scolds her for keeping a secret from her parents that is causing them to fight again; he blames her for their fight. Things were starting to improve between their parents' relationship.
Aria and Mike had agreed to make dinner for the parents as a way to a family date night, but due to the argument, Ella cancels, leaving the three to eat dinner alone. Mike is very upset, as he wants his parents' relationship to stabilize again, or better yet, go back to the way it was before.
Season 2
Even though Mike was the one who wanted his parents to get back together the most (and for Ella to come home), he is not contented with the situation and upset by how much has changed. Ironically, Aria is the one enjoying the return, not Mike.
Mike starts picking fights at school, one of which is stopped by Aria and Sean Ackard. Aria scolds Mike for fighting with kids bigger then him.
In Blind Dates, Aria goes to find and pick up Mike at the basketball cage, as per her mother's request, but only finds Mike's friends and even, Jason. His friends let her know that Mike hasn't been around in awhile, and Jason theorizes that Mike is doing things his family obviously wouldn't approve of.
When Mike comes home, he scares her by not speaking up when she asks who's there. She also confronts him about his lies, but he brushes her off, as he is not dealing with his transitioning family situation very well.
Aria's dad picks up on Mike's abnormal behavior at home, but Aria denies what she knows, having been threatened by Mike that he would tell their parents about her hanging out with her friends.