Marvel's The Defenders: Every Character Confirmed So Far
5. Trish Walker
Confirmed By: An official announcement on the show's Twitter account.
Plot Predictions: This could, like many of the supporting characters, just be a small part - seeing Trish as Jessica's best friend, which could be used for showcasing the pressures she's now under, giving exposition about working on the case/with the other Defenders, and just serving (as many of them will) as part of the general world and character building.
On the other hand, it's expected that Trish will take on her comic book identity Hellcat eventually. The character will be back for Season 2, so it could happen then, but Hellcat has often been a member of the Defenders in the comics - more so than the likes of Jessica Jones and Daredevil.
If they wanted to establish her as a hero, then there's a chance to start building towards that by having her getting more invovled with The Defenders (she started training more to fight as JJ went on), laying groundwork for the full transformation in Season 2 of Jessica Jones.