Riot Games stated that Valorant will get two additional agents and one map by the time the game sees full release. So far, it looks like Sabine, the Vampire is the closest one to a finished state.
Valorant launched into closed beta with 10 agents on its roster. During the play sessions with influencers, there were only eight, with Breach being added at some point during those sessions and Raze at the launch of closed beta.
It seems like Riot Games are looking to keep the momentum of adding new agents as they also stated that a total of four maps and 12 agents will be available at Valorant's full launch.
As you may have guessed, this prompted data miners to look into these claims and they found evidence that points to Venice being the new map as well as Sabine, the Vampire being a new agent.
So far, there is only a single spoken line by the new agent, along with her icon. You can hear and see them at the start of the video below. However, the existing agents keep referencing her in their voice lines.
For example, Breach notes that "The Italian has some fancy toys" that he wants to break while Viper and Omen want to find out Sabine's secrets. The latter also references the monstrous nature of them both but notes that Sabine at least still has "flesh to hide her horrors".
Cypher seems to have found these secrets and also has a voice line where he tells his team that "she can only suck [their] souls out after [they] die", hinting that the agent will be able to exploit enemy corpses for some benefit. Interestingly enough, Cypher can do the same with his ultimate when he throws his hat at a dead enemy to find the positions of all their teammates.
Keep in mind this is all datamined info and as such may not always lead to accurate assessments.