The 31-year-old Slovakia international has penned a 10-week contract with Danny Lennon's side just 18 months after walking away from Tynecastle.
He was frozen out by former owner Vladimir Romanov after turning down a move to Austria Vienna, but insisted he still had a soft spot for the Jambos.
However, when it was pointed out to him that by helping struggling St Mirren survive in the top flight this year he could help relegate Gary Locke's men following their 15-point deduction for going into administration, Kello refused to even consider that dreaded fate.
He said: "I don't want to think like that. I will try my best for the club I am at now. That is all I can do.
"It's not nice or easy to see Hearts where they are. This club and its supporters deserve much more. Unfortunately they are they and they need to go with the future and hope it gets better.
"I have sympathy with what the players are going through at the club. There are so many talented players there and it's such a shame they are in this situation. There is a lot of pressure on them and it's not fair because they deserve to be in a better situation."
Kello negotiated an early release from his Hearts contract in April last year and went on to have spells with Romanian side Astra Giurgiu as well as Wolves in England's Championship.
However, he was not offered a new deal with the Molineux side and then failed to win a move to Doncaster following a trial.
But Kello - who also played with Romanov's Lithuanian side FBK Kuanas - arrived in Scotland on Wednesday to answer an emergency call from Lennon after he lost keepers Chris Dillo and Kieran Hughes, leaving him with just on-loan Swansea stopper David Cornell to fill in between the sticks.
He could now feature against Dundee United at Tannadice on Saturday and is hoping to earn himself a longer stay in Paisley.
He said: "Danny told me he needs help in the goal and I suppose I am going to play. He never said I would play for certain but I came here to play so I can prove myself and help the team.
"This is a chance for me to get a longer-term deal either here or somewhere else. Hopefully we can get some results and move up the table. After that we will see what happens."
Lennon was delighted to get Kello on board, although he admitted the decision to bring him in was not his to make.
He said: "He's a player that has been on our goalkeeping coach Paul Mather's radar for a while. Paul is in charge of that department, picking goalkeepers. He knows more about that position than I do.
"I can be sitting with Paul watching a game and say: 'What a save that was'.
"But he will see something like the keeper going with his wrong hand or something else and pick faults in it. It's not something I profess to be an expert on but I'm working with one of the best goalkeeping coaches in Scotland and I know he will get Marian back to the levels he was at before."