Arc is a mysterious young man dressed in a cape and fedora.
He first meets Yucie when he seeks shelter at a church at which she is temporarily working. It turns out that he is being sought by the castle guards, which alarms Yucie. Although Yucie is very saucy towards Arc, she finds herself drawn to him. Arc heroically saves Yucie when they encounter a demon from the Demon World. He also calls her "Huge Forehead" to tease her about her big forehead, much to her consternation. After kissing her on her forehead a few times, he finally gives her a real kiss, which is witnessed by her friends. Arc is also hunting for the one that first put together the Eternal Tiara, which he feels will lead him to his real father. In reality, he is Prince Arrow, son of Queen Ercell. (This is in fact the reason he is being sought by guards, as he escaped from the castle to find his 'biological father'.) At the same time, he is the prince that once saved Yucie when she was in danger when they were small. Later on, it was shown that Arc was also drawn to Yucie.
At the end of the series, it shows Yucie is a wedding gown in her 17 year old form. It switches over to Yucie and Arc being pronounced Husband and Wife. Yucie closes her eyes expecting a traditional kiss on the lips when Arc instead kisses her on the forehead as he has done throughout the series. However all of this is only a dream and of course Yucie wakes up from it. She gets dressed, is called to school a centimeter taller then usual (as shown on her growth chart), and goes of presumably to make her dream a reality.
When he was younger, he rescued Yucie from a dog. Ever since then she has been looking for him to thank him. Over the series they become more romantically involved. Arc is often the one to save her from whatever trouble she gets into.