Of all the colors, this is probably the most difficult to understand as it applies to a sapient individual, but heroes of destiny and their mentors tend to have at least some green in them
You dont have to find any other life gain cards since he can give your other creatures lifelink, but he cares about creatures gaining life as a means of bestowing +1/+1 counters
If the card is a creature, it will have its power and toughness in the bottom right
While controlling another player, you make all choices and decisions the controlled player is allowed or forced to make for the duration, and all information available to them is available to you (usually the cards in their hand and any face-down cards they can see)
In the early game, you can use it to recur a key creature, while in the late game, you can leverage both sides for maximum value