Originally Tails the fox was Sonic's friends but was being corrupted by The White Demon and Tail's turned evil because of that.
Tail's becomes more evil, Corrupted, tougher, bigger, mature, smarter and greedy that ever.
His good side has left him and was converted into the evil side. The most fearsome being. Tails is a good person but was being Corrupted by The White Demon and becomes evil, Corrupted and greedy. Hates humanity. Loves his friends and still have sympathy. Very polite, clever,loyal