[2] He attended Patrician Brothers' College, Fairfield as well as Westfields Sports High School and played his junior rugby league for the Guildford Owls.

Fainu pushed ahead to the Jersey Flegg Cup (Under 21s) later in the year whilst still eligible for Under 19s, playing seven games with three tries including a double on debut in round 14.

Fainu made his first grade debut from the bench in his side's 12−6 victory over the Wests Tigers at Campbelltown Stadium in round 8 of the 2023 NRL season.

[4] In July 2023, Fainu signed a four-year deal worth $2 million to join the Wests Tigers ahead of the 2024 NRL season.

[5][6] Fainu played 23 games for the Wests Tigers throughout the 2024 NRL season as the club finished with the Wooden Spoon for a third consecutive year.