Can i use gta v story mode characters in gta online?

Quick Answer
No, characters from GTA V's story mode cannot be used directly in GTA Online; the online mode has its own separate character system.
Explanation
Grand Theft Auto Online and the single‑player story mode of Grand Theft Auto V are distinct game modes that share the same open world but run on separate systems. In story mode you create and play as one of three protagonists—Michael, Franklin, or Trevor—while in GTA Online you control a customisable anonymous avatar with no direct link to those story characters. Rockstar has not provided any means of transferring story characters into the online environment. Consequently, players cannot spawn as or inherit the appearance or attributes of the story mode protagonists in GTA Online. The two modes do share some assets like missions and vehicles, but the character models and progression systems remain separate.