Yūichi Haga, is a member of the Spookies, known simply as "Yū-ichi" among them.
Childish and friendly, he can sometimes be annoying. Six tends to yell at him for being childish and klutzy. Despite his cheerfulness, he is more sensitive than he appears.
Even though Six is a year younger than Yūichi, he treats him like a child most of the time. As the only one in the group without a codename, he is quick to point that his name is "cooler" than any nickname. He grew up in a wealthy family and is a single child, making him wish he had a sibling.
He gets into an argument with Six at the Auto Plant, where Six tells him to get his act together and stop being a klutz, leading him to leave the facility. When he arrived back at base he read Finnegan's post on Summoner Net, and in order to prove himself heads for Amami Float. He gets captured by Finnegan and loses his hat which is eventually found by the protagonist.
After the battle against Finnegan, they find him safe and sound. However, he is acting strange and far less emotional than usual, not caring that Six is insulting him or when Spooky is trying to advise him not to act recklessly. He goes home after the incident.
He later follows the Spookies to Algon Microelectronics where after Lunch and Spooky crack the main computer's defenses they find the real purpose of the Krypto Chip installed in every computer in Amami city is to collect souls for Manitou. Luckily, Lunch replaced the Krypto Chip inside their trailer's computer with the prototype he stole from his father earlier in the game.
Yu-ichi later devises a plan to reveal their new-found knowledge to Amami city via Algon's airwave which backfires and ends with all the Spookies except Leader (and strangely, Hitomi) branded hacker terrorists, which brings Yu-ichi to the conclusion that Leader is a traitor and ratted them out with Kadokura's message to Leader being additional proof. Yu-ichi, Six and Lunch then leave Spooky, with the option for the player to do the same.
After Malsumis is defeated, it is revealed that Yūichi was brainwashed to work secretly for the Phantom Society as their messenger and was the real traitor. He is kept under watch by Rei Reiho while the protagonist and Nemissa head off to the Amami Monolith to rescue the others. In a New Game Plus on the PlayStation/3DS versions, if the correct choices were made to save Spooky, Rei and Yu-ichi later head to the Monolith where they recover him from the roof.
At the end of the game, Yu-ichi leaves the Spookies and vows to become a "super hacker" like Spooky was, and Six supports Yu-ichi, showing their rivalry has somewhat decreased.