Taki is one of Haru's best friends.

They know each other since they were little. 

Since a little child Taki is sitting in a wheelchair. As a child he was a bright, open-minded kid and became friends with Haru.

Unfortunately due to several hospital stays, Taki moved away after elementary school.

This was a great loss for Haru, who naturally had a hard time making good friends. 

Due to coincidence Haru and Taki met again in Highschool and still get along swimmingly.

Taki has grown into a young, funny, sarcastic man - although he rather refers to himself as 'realist'.
He isn't rather optimistic about Haru and Josh becoming a couple and sees himself in a competition with Lena over their friendship with Haru.

Taki is the only one having many friends outside of the trio. He tries to do everything by himself and dislikes being helped. 
His mum is rather over-protecting towards him, not realizing, that he just wants to grow up like everyone else.

Relationships

Haru and Taki are childhoodfriends and have many inside jokes and shared memories. 
They are both airheaded at times and talking about trivial nonsense - which is annoying Lena from time to time.

Lena and Taki met in Highschool for the first time. Despite their usual fighting, they get along pretty well.

Taki actually even has a little crush on Lena. 

Josh is an always charming nice-guy, which Taki absolutely cannot stand. 
In fact, Taki is the only one not trusting Josh at all. He is suspicious whether Josh is actually just toying around with Haru.

He refuses to accept help from Josh and occasionally gets jealous, because Lena is also hyping Josh.

Mr. Parker is Taki's homeroom teacher and making sure that Taki can follow the class. Taki and Mr. Parker are always throwing ironic jokes at each other.