Solarie of Astora
Often times referred to as "Sunbro" this knight has become something of a iconic symbol for co-operative play in Dark Souls. Although it is possible to fight against him, players who don't will reap huge benefits should they chose to seek his aid in several difficult boss fights. Supposedly, the lore of the setting also subtly implies that Solarie is actually an younger relative of the last boss. Summoning him for the last battle makes for an interesting case of kinslaying and regicide via player assistance. It's true that in the above video he is defeated, but you must bear in mind that he abstains from using magic equipment and instead prefers to rely on his innate abilities (yes, that includes the divine gift of being able to throw lightening bolts).