Schareina was born in Valencia to a German father, Torsten Alexander Schareina from Braunschweig near Hanover, and a Spanish mother, Mara Marzal. He developed an interest in motorcycling through his maternal uncle Mariano, who competed at regional level in motocross events; as a child Schareina practiced on the circuit in Paterna, the Valencia-area town where his uncle lived.
At 20, in 2015, he won the Spanish Cross Country Championship in the Junior category riding a Sherco, then signed his first professional contract with Husqvarna and added the Senior championship the same year. He represented Spain in the International Six Days of Enduro (ISDE) for four consecutive years, building the technical skill base that would later serve him in long-distance rally navigation.
Schareina moved into rally-raid competition in 2020, winning the "Road to Dakar" category at the Andalucia Rally and earning direct entry into the following year's Dakar. He debuted at the 2021 Dakar Rally on a KTM, finishing thirteenth overall and second-best rookie, while also winning the Marathon category. He could not compete at the 2022 Dakar due to a late registration, but won the Baja Aragón that year.
After joining Monster Energy Honda HRC in 2023, he described winning the Desafío Ruta 40 in Argentina as the moment things changed: "Mexico 2023 was the beginning of everything. It was my first race with Honda and my first podium in the World Rally-Raid Championship. I went to Argentina much more confident and it was an incredible thrill to take the win." That season he also won the Baja Aragón again and placed second at the Sonora Rally, finishing thirteenth at the Dakar for the second time.
Schareina entered the 2024 Dakar as one of the favourites, winning the prologue in Saudi Arabia before a crash on Stage 1 fractured his wrist and forced his retirement. The same injury ruled him out of the Abu Dhabi Desert Challenge, effectively removing him from W2RC title contention.
He rebuilt his season from the middle rounds: he won the Rally of Portugal with three stage victories; finished second at the Desafío Ruta 40, winning the prologue and two stages; and took second at the Rally of Morocco, again winning the prologue and two stages. He finished the W2RC season third overall. In parallel, he won the FIM E-Xplorer World Cup for electric motorcycles in the championship's debut season.
The 2025 Dakar, run across Saudi Arabia over 7,453 kilometres and more than 53 hours of competitive racing, proved the hardest of Schareina's career. The edition included the 48-hour Chrono stage — a two-day, 1,000 km challenge with no mechanical assistance and a night under the stars — which Schareina navigated while lying second in the overall standings from Stage 4 onwards.
On Stage 5, the day before the rest day, he crashed and fractured his collarbone. He chose to continue and did not disclose the injury publicly until after the race had finished. A second, more dramatic crash on Stage 9 was captured on video and shared widely; he remounted without worsening the injury. On Stage 11, the penultimate stage, he took a stage victory.
The final stage used a mass start reminiscent of the early Paris-Dakar format. Schareina crossed the finish line 8 minutes and 50 seconds behind winner Daniel Sanders, with teammate Adrien Van Beveren completing a Honda podium sweep. He later reflected: "I knew it was going to be seven days of a lot of suffering but we had to keep fighting for the victory until the last kilometre."
Following his Dakar podium, Schareina won the 2025 Rally Sertões in Brazil on only his second appearance at the event, dominating across six valid stages by winning five of them and taking both the overall title and the Moto 1 category. He placed second at the 2026 Dakar with three stage victories, and second at the 2025 Abu Dhabi Desert Challenge after winning the prologue, Stage 1, and Stage 3 before losing the lead.
Dakar finishes: 13th (2021), 13th (2023), retired Stage 1 (2024), 2nd (2025), 3rd (2026). Other titles: Baja Aragón (2022, 2023), Desafío Ruta 40 (2023), Rally of Portugal (2024), FIM E-Xplorer World Cup (2024), Rally Sertões (2025). His Honda CRF450 Rally is prepared by Monster Energy Honda HRC and competes in the Rally GP class of the W2RC.