Ark is a rookie Gate Guardian assigned to work under Jester of the Six Stars. He is somewhat cocky and overconfident in his abilities at the start, however throughout the game he grows into a more mature, yet still confident, leader. The Gate Guardians Motto, "Always Protect Everybody," is practically his mantra.
Not much is known about Ark's past. Considering the plot was more about how he worked to protect everyone and get them home safely, that's not surprising. All we do know is that he joined the Gate Guardians a month after Ivy did. Unless this promised sequel to CIMA comes out, and it brings something up, we'll never know for sure.
As far as his love life goes, Ark seems to have his choice between two girls as of now. He does blush when Philis gets affectionate in the dungeon you save her, however he gives no reaction to her confession of love during her 100 Trust Conversation. At the same time, even though they fight (anime romance, anyone?), Ark does express great Trust in Ivy from early on. And in the end...well, let's just say Trust hit an all time high before the final battle and Ark's reactions/words during and after that scenario were moving.
While there is no resolution for Ark's feelings on either woman in the game, again waiting for a sequel, my money's on Ivy as of now.