He has a more stoner face that no one would take seriously, he seems very mocking.
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It reminds me of the boy from Scary Movie
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Blake as an antagonist, that's fine. Even if the idea is for him to be stoner and carefree, even immoral, Blake should have some notable trait that makes the protagonist feel insecure.
For example, a good contrast would be for the protagonist to work hard to earn money, but his friend works little but earns the same or more. Inviting them to dinner would be a good idea, and if you want to include a post-celebration scene, like the protagonist drinking a lot and passing out (NTR moment).
Another scenario is for him to bring different girls home, which could lead to Cristina seeing him having sex and sparking her curiosity.
You could have Blake have hobbies that catch Cristina's attention, like being a Big Brother. Although he's very laid-back, he'd like to be a father at some point. She's a teacher, so taking care of others could be something she enjoys.