An off-duty security guard who died at a Melbourne train station on Friday night is being remembered as a "quiet hero" by his friends and colleagues.

Aidan Becker, 22, was allegedly stabbed to death after intervening to help a 14-year-old who was being robbed by four teenagers.

His friends said they were heartbroken by the loss of their mate who always put himself before others.

"He was the type to go out of his way and do what he did, and that's probably the biggest thing I love about him," his friend JD said.

"It's just heartbreaking - I miss him."

The spot where he died has turned into a shrine to the brave man, with members of the public leaving floral tributes throughout the day.

"It's just every parent's worst nightmare," local resident Rachael Taranto said.

"We really want the family to know we're all thinking of them, and their son is a hero."

Becker was a local to the area, who loved animals, travelling, and making music.

He worked at the Alfred Hospital, which released a statement this afternoon mourning his death.

"We are deeply saddened by the passing of young security officer, Aidan Becker," it said.

"Aidan's colleagues at The Alfred describe him as a dedicated, respected, and much-loved member of our security team.

"He will be remembered for being full of life, and for his positive energy, kindness and dedication.  He was a quiet hero who gave his all for those around him."

Two 17-year-olds, a 16-year-old and an 18-year-old were all arrested by police.

The 16-year-old, a 17-year-old boy from the Whittlesea area, and the 18-year-old were charged with murder and armed robbery.

One of the 17-year-old boys is still in hospital under police guard, and has not been interviewed or charged yet.

The 18-year-old man appeared at Melbourne Magistrates Court today and did not apply for bail.

He will remain behind bars until his next court hearing in June.

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