Helenius, 18, was selected in the second round, 59th overall during the 2021 NHL Draft after trading back into the second round by sending a third and fourth-round pick to the Carolina Hurricanes. The 6-6, 201-pound forward appeared in his first season in Liiga, the top league in Finland, where he posted 14 points (7-7=14) in 54 games during the 2020-21 season. He appeared with Jokerit U20 (U20 SM-Liiga) in the 2019-20 season, registering 24 points (13-11=24) and a plus-14 rating in 51 games.
The Jarvenpaa, Finland native represented his home country at the 2021 IIHF World Junior Championship, posting four points (2-2=4) and a plus-2 rating in seven games to help Finland win the bronze medal.