Blaine has a twin he doesn't know about, but that twin knows him, and more importantly, Kurt. Based on (http://blartinas.tumblr.com/post/32052611026/au-in-which-blaine-doesnt-know-he-has-a-twin) this gif set.
Blaine was walking down the hall with Tina and Artie, discussing their set list for sectionals, when Artie suddenly froze in the middle of the hallway, almost causing Blaine to trip over the boy’s wheelchair. “Artie, what are you-”
“Shh!” Artie hissed before wheeling to the outskirts of the hall, taking refuge by the nearest lockers and motioning to Tina and Blaine to follow him.
“What-” That’s when he saw him. “What. Is. That?” Blaine stood, frozen in shock and confusion, as he watched a figure strut down the hallway. He was wearing red high water pants, a tight black polo, and a bowtie. Blaine felt the breath catch in his throat. The boy had his face. The same thick eye brows above the same set of honey colored eyes. The same dark hair that was gelled down in the same fashion. Blaine felt like he was going to throw up as the boy walked over to Kurt, Blaine’s boyfriend.
Kurt gave his hair one last spritz of hair spray before placing the cap back on the bottle and placing it in his locker where he’d most likely need it again after lunch. He then grabbed his pre-calculus book and moved to close his locker, but jumped a little when he noticed he wasn’t alone; there was a familiar boy standing beside him and Kurt let out a breath of relief at the sight of his boyfriend. Kurt snorted a little at the boy’s appearance. “Why are you dressed as if you were in a boy band?” The boy glanced down at his clothes before smirking back up at Kurt.
“I thought I looked pretty hot. Although, now that I’ve seen you, I know I’ll have to try harder next time.” Kurt felt the blush creep to his cheeks and he rolled his eyes. He closed his locker and turned to face the boy completely. The boy looked him up and down until his eyes finally rested on Kurt’s. “A lot harder.” Kurt cleared his throat slightly, not accustomed to Blaine’s bluntness. At least not freely in the hallways of their very homophobic high school. Kurt glanced around nervously before looking back at Blaine with a flirty smile, deciding it was relatively safe.
“You don’t have to try at all,” Kurt said, taking a step closer to the boy and noting that he smelled different; he wasn’t wearing the cologne Kurt had bought him for his birthday. Blaine wore it every day. He must’ve run out. His boyfriend looked at him with a hungry expression and wrapped an arm around his waist.
Blaine could feel his blood boiling in his veins. He didn’t know what the hell was going on, but he would break that boy’s hand for touching Kurt. Blaine stormed off toward the two and pushed his doppelganger away from his boyfriend. Kurt looked at Blaine with wide eyes and then his jaw dropped and he looked between the two identical boys in confusion. “Blaine?” he asked, clearly puzzled, but Blaine was too busy.
“Who the hell are you?” Blaine growled. “Why the hell do you look like me? And what the hell are you doing with Kurt?” The other boy smirked at Blaine, seemly unfazed by the burning rage in Blaine’s glare, and wiped off the front of his shirt.
“Relax, Blaine, Kurt and I were just talking.” He held out his hand to Blaine who simply narrowed his eyes at the boy. “I’m Michael.” They boy glanced down at his still outstretched hand and back up to Blaine. “C’mon, is that anyway to treat your brother?”