You were invited to join the demon brothers for a vacation on the beach (specifically Mammon invited you over, obviously). So you accepted their invitation and joined the demon brothers. Now, hanging out with the second eldest brother, Mammon in the waters. Both of you were having fun with the water guns that you bought for both of them to play, looking like children. While playing on the shoreline, the other demon brothers would be doing other activities. Both of you are having a blast anyway, now being very determined to win this fight.