The defender only joined in 2021 but could finish the season with a Premier League trophy in his second campaign at the Emirates.
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And the Japan international, though not always considered first-choice, has proved vital in defence for Arsenal and is able to play in every position.
But while the 24-year-old has already faced some of the biggest and best strikers in the Premier League, one forward terrifies him more than anyone - his own team-mate Gabriel Martinelli.
When asked who gives him the most trouble in training, Tomiyasu told Goal: "Gabriel Martinelli. He never stops.
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"He has pace, aggression. He's still young, but when he has the ball he wants to make the difference, he wants to do something."
Martinelli has taken on a bigger role in the squad since Mikel Arteta arrived as manager of Arsenal, becoming the main man on the left flank.
This season, the Brazilian has seven goals in 19 Premier League appearances - already the best tally of his career - while the 21-year-old forms a youthful attack that also includes 21-year-old Bukayo Saka, 24-year-old Martin Odegaard and Gabriel Jesus, who at 25 must feel like a veteran.
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Arsenal currently lead Manchester City by five points at the top of the table while Arteta has been hailed as the Gunners look favourites to win the league.
"Mikel knows about football, he knows about football structure, [he knows] everything," Tomiyasu added.
"Our job is to just explain on the pitch what he says. I believe him. He's just amazing.
"He's the best manager I've ever played with.
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"We understand more [about] Mikel's football.
"We've got depth, we've got some new players, but the biggest thing is we understand Mikel's football more than last season."
Arsenal face Everton next on Saturday, who have just appointed Sean Dyche as manager but face a huge task staying in the Premier League.
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The Gunners, though, could go eight points clear and still have a game in hand as City take on Tottenham on Sunday afternoon.