Mark Ruffalo plays the Hulk as well as Bruce Banner in Spider-Man: Brand New Day. The character is the physically powerful alter ego created by Banner's exposure to gamma radiation.
Banner uses an inhibitor device to suppress the gamma radiation in his body and maintain his human form. When that device is removed, he changes into the more destructive Savage Hulk persona. The film deliberately presents this version as frightening and uncontrollable.
This is different from the more integrated Hulk state seen in some earlier MCU stories, where Banner and the Hulk shared a merged personality. Brand New Day draws on the classic contrast between the rational scientist and the enraged green humanoid. Peter seeks Banner's help because he is experiencing a similarly unsettling evolution in his own powers.