Marly Spears-Fletcher

 is the adopted brother of Phineas and Ferb. Marly is a couple years younger then Phineas and Ferb and was adopted by the Flynn-Fletchers.

Personality
Marly is very kind, especially to his friends. He is always willing to have fun with any of his friends or family.

Phineas
Phineas is Marly's closest brother. When Marly first came to the Flynn-Fletcher house, Phineas welcomed him and gave him a tour. Whenever Marly is unhappy he will come to Phineas for comfort.

Ferb
Marly isn't very close to Ferb, but loves him as a brother. Marly helped Ferb in the extended version of The Night before Phineas (The Night be-Ferb Phineas) while Phineas helped Pedro.

Candace
Marly doesn't completely understand Candace but looks up to her and enjoys her company. Though Candace is often annoyed by the fact that Marly is always around her, she secretely enjoys his company.

Isabella
Marly likes hanging out with Isabella and Isabella called him "cute" on one occasion. In the extended version of Isabella's Christmas Wish (A Rollercoaster Wish), Marly comforts her and Isabella hugs him.

Buford
Marly dislikes Buford and thinks he is disgusting and annoying.

Baljeet
Marly and Baljeet are very close, as they are close ages. Baljeet teaches Marly in The Night be-Ferb Phineas and enjoys his company.