Between Issei Hyoudou and Rias Gremory and Akeno Himejima and Koneko Toujou and Yuuto Kiba and Mysterious Transfer Student (original character) as siblings.
The serene morning air of Kuoh Academy was occasionally disrupted by the high-spirited shouts and laughter of students eager to start their day. Issei Hyoudou, ever the lecherous yet good-hearted devil, was among them, strolling through the sea of uniforms with a none-too-subtle gaze lingering on the female student body.
Today, however, the usual routine was broken by whispers about the new transfer student, a person shrouded in an aura of mystery—one who oddly kept out of the limelight and away from the usual high school social circles.
Issei's attention, however, was not on rumors but fixated on a far more earth-shattering problem: Rias Gremory's approaching birthday and his lack of a suitable present for his bewitching club president. It wasn't until the Mysterious Transfer Student accidentally bumped into him, an unintentional but firm shoulder check, that Issei finally took notice.
"Whoa, watch where you're going!" Issei exclaimed, ready to launch into his standard tirade, but the words caught in his throat. The student – with eyes reminiscent of twilight skies – simply nodded in apology and continued on their way, leaving Issei perplexed, feeling an inexplicable connection he couldn't quite place.
Lunchtime found Issei at the Occult Research Club, his encounter briefly forgotten as he, Akeno, Koneko, and Yuuto listened to Rias outline their latest supernatural concern. Yet the door creaking open interrupted their discussions, revealing the same Mysterious Transfer Student standing at the threshold. Rias, with the grace befitting her stature, welcomed the newcomer, introducing them as a new member interested in the unexplainable aspects of the world.
Days transitioned into a series of peculiar coincidences and contests. The Mysterious Transfer Student displayed skills and a demeanor that cut through Issei's reservations, leading them to victories in otherwise evenly-matched battles against rival schools and creatures from other realms.
One afternoon, Issei returned home, flipping through a box of old photographs and heirlooms passed down from his grandfather. Hidden at the bottom, an ancient letter tied with a red string called to him, its seal resembling that of his own Sacred Gear.
With barely concealed anticipation, Issei broke the seal. Inside, he found a message, its script dancing before his eyes, speaking of a celestial pact made long ago. At the end, it cryptically alluded to a person of shared blood and destiny, who would reveal themselves under the sign of the red dragon.
Jolting upright, Issei's mind raced with possibilities. The similarities, the skills, and now this letter—it couldn't be a coincidence. Could it be that this Mysterious Transfer Student, who had unconsciously slipped into his daily life, was actually his sibling? A bond created in shadows, waiting to be brought to light?
As the night closed in, Issei knew that the answer would entangle his life in a web of secrets he never imagined, shaking the very foundations of his existence and the supernatural world he had come to know. With this newfound realization heavy on his heart, Issei knew one thing for certain—the next club meeting would be the start of a life-altering revelation.