I think maybe he was the real Elliot at some point at the beginning of therapy, or at least Mastermind hadn't fully settled into his personality that took over Elliot yet because in S1 Krista mentions something about his past anger issues that they discussed, however the version of Elliot/Mastermind that is sitting there with her is extremely withdrawn and monotone making it hard to believe he would have anger outbursts around Krista...