Koa Rey

Koa Rey is a drug rehab councelor and the brother of current Five-0 member Tani Rey.

Season 8
Koa was introduced as Tani's delinquent younger brother. His sister's teammates help procure him a job as a construction worker to better help him keep his life in order. Koa presents himself to work, but continues to use opioids until he suffers an overdose. He is found unconscious by Adam Noshimuri, who rushes him to Noelani.

He survives the overdose and is confronted by his sister. After a tense conversation, he promises Tani that it won't happen again. Tani says his word isn't enough anymore and Koa is persuaded to enter inpatient treatment for his addiction at the Island Hope Rehab Center.

In 8x22, Koa has successfully completed his treatment. He exits the facility and hugs his sister, who is there to pick him up. Tani happily remarks that Koa looks a lot better. However, Koa tells her he isn't leaving: he's been offered a position as a rehab councelor and has already said yes. He apologizes to Tani for making her drive all the way there for nothing, but wanted to see her after such a long time. Koa is later seen leading a support group, where he talks about the importance of love and support in the road to recovery.

Season 9
Koa is still working as a councelor at Island Hope. He gives Tani information to help find a missing girl called Alanna. One of the people Tani interrogates in her search for her refers to Koa as "Mr. Rehab", deeming him straight-edged and boring.