Proto Man. When you reach the end of Proto Man's castle, you'll be weakened to your last hit-point, Proto Man then shows up, saves Mega Man, reveals the other Proto Man is simply Dark Man 4, and he gives Mega Man an L-Tank.
Dr. Cossack gifting Beat to Megaman as an apology for what happened in the previous game.
Dr. Cossack's assistance in general, really. Mega Man is stated to have the general intelligence of a child. Having your parents (or parent in this case) suddenly stop being around can be a very frightening experience, leaving Megaman and Roll alone with Rush. Dr. Cossack however comes through as a person to repair and care for them in Light's absence.