Considering that Yuya and Yuzu are 14, I'm presuming the same for Yuto and Ruri. As such, they are 2nd year students at Heartland Academy. This is before the invasion so they are leading peaceful lives.
Also headcanon since I like to imagine that Yuto was, in fact, quite shy before the war started (silent and shy XD) while Ruri was caring of her friends if not a bit teasing when it comes to her best friend.
Cute
"Yuto!"
The 14-year-old teen looked up from his D-Gazer and turned around to see his best friend and secret crush, Kurosaki Ruri, running towards him with a big smile on her face as she waved at him. He was about to wave back when he sweatdropped, noticing Shun's glare as he followed after his sister quite closely. He was the sort of overprotective older brother, after all.
"What's wrong?" Ruri questioned, oblivious to her brother's behaviour since her back was turned to him. He shook his head.
"I'm fine..." he began when the girl's look became somewhat sly.
"Were you dazzled by my looks?" she teased, causing Yuto to blush as red as a tomato as she laughed, grabbed his arm and began to run with him, ignoring Shun's shouts for them to come back to him right now.
"Ruri! Shun will get angry!" Yuto yelled out as they gained the attention of a greenette, who was walking to school with that strange boy who liked his necklace a bit too much. The girl just giggled as they raced past them, but he managed to catch sight of the boy complaining how people need to be more careful and something about a 'Shark'. "And kill me!" he finished.
"Nah, Shunnie won't kill you." Ruri chuckled. "You are too cute for that!"
"W-What?" if that was even possible, Yuto's face went an even deeper crimson.
His friend... crush... whatever just laughed again as she took him to the local park as Mr. Heartland announced this year's World Duel Carnival.
This was meant to be a simple YutoxRuri drabble, yet ended up being that and cameos! XD
Anyways, thank you for reading! :)