Even if Rena's personality is "bad", it's good that it's flawed. You guys say like it's easy to make up and be friends but it really isn't, especially for a strong personality like Rena. She said things out of impulse and the fact that she feels she has words she can't take back, add to the fact that she feels like a burden to the team because she wasn't consistent in her practice, leads her to avoid it and believe it's better to not have her rather than a baggage.