Sword Art Online may be about Kirito’s adventures navigating threatening virtual realities, but everyone knows it's also a shameless harem anime. Though Asuna is the only canon girlfriend Kirito has had, practically every girl he comes across seems to develop feelings for the talented swordsman. However, the very first person to show interest in Kirito isn’t a girl -- it was Klein, his first SAO companion.
Hitting it off almost instantly after meeting, Klein and Kirito form the first Sword Art Online friendship, and they remain close comrades throughout the series. While some may argue that Klein has only ever shown friendly love for Kirito, there’s strong evidence to suggest he may have had romantic feelings for his game partner as well. There's a crowd of girls smitten with Kirito, but Klein may have been the founding member of his ever-growing harem.
When Klein first joins Sword Art Online he knows very little about the game but is determined to learn the ropes. The outgoing video gamer quickly identifies Kirito as a beta tester and asks the experienced player to teach him the basics of fighting. The two form a bond while training, compelling Kirito to consider him a long-term SAO partner even though they just met that day. When Akihiko Kayaba announces that players who die in the game will die in real life, Kirito comes up with a plan for the two of them to run off together and survive side by side.
While Klein seems impressed by Kirito’s plan, he declines the invitation in order to help his other friends survive with the knowledge he learned from Kirito. As a former guild leader, Klein assures him he’ll be okay and Kirito turns to leave. However, before he goes, Klein calls out to him to exchange some heartfelt parting words. He shifts nervously and stammers before professing, “You have a pretty cute face,” also adding, “You’re just my type.”
After Kayaba transformed the players from their avatars to their real-life appearances, Klein makes a point to acknowledge Kirito’s looks and express how much he likes him. It could be argued that Klein’s coy comment is a joke or that it merely spurs a sort of “bromance” relationship, but the moment is undoubtedly sweet and confessional. Moreover, Kirito’s kind reaction is just as congenial as his response to the girls who get close to him -- save for Asuna, of course, to whom he expresses his feelings overtly.
Sword Art Online's English dub does change the dialogue between them to sound less flirty, Klein saying Kirito looks “cool” and prefers his real face to his avatar. However, English dubs are notorious for censoring gay content. Sailor Moon’s Neptune and Uranus, for example, were made cousins instead of romantic lesbians. SAO’s Klein X Kirito moment is obviously much more subtle, but erasing those queer vibes does raise some eyebrows.
It’s also notable that the flirtatious moment they share kickstarts a never-ending trend of people admiring Kirito. The harem builds after Klein, with following girls Sachi, Silica, and Lisbeth also having an attraction to Kirito. Klein shows interest in many girls as well, but Kirito's feminine look may have legitimately attracted him as much as the ladies who followed his lead.
Though Kirito and Klein spend a long time apart as they navigate Sword Art Online separately, they hold on to that first friendship and care about each other until the end and beyond. Klein constantly worries about Kirito’s dangerous mentality to go solo, always urging him to join his guild to be safer. He has shed tears multiple times when believing Kirito to be in danger. It’s certainly possible that this strong friendship could be connected to Klein’s deeply rooted romantic feelings.