Sunlight Medals: These are earned when you summon an NPC to help you on a boss fight. If you defeat the boss and your NPC friend is still alive, you will earn a Sunlight Medal. These are also obtained in New Game+ if you have 50 Faith and defeat a boss while human.
Sorry, my original post was vague about this. I just meant you had to be human so you can summon Soltaire and obtain the Medal. There are also 3 sunlight medals in a chest in Anor Lando, and you get a sunlight medal for freeing Lautrec.