Los Angeles, California - Convicted rapist Danny Masterson was a "role model" to his That '70s Show co-star Ashton Kutcher, actor told a judge according to a letter that emerged Friday.

Masterson was sentenced to at least 30 years' prison for drugging and sexually assaulting his victims at his swanky Los Angeles home two decades earlier.

In a submission to the sentencing judge, Kutcher said the prominent Scientologist had been "nothing but a positive influence" on him.

"I met Danny Masterson when I was 20 years old in 1998," he wrote. "He instantly became a friend, dedicated co-worker, and role model to me. And has remained as such for 25 years."

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"He's an extraordinarily honest and intentional human being. Over (the) 25-year relationship I don't ever recall him lying to me. He's taught me about being direct and confronting issues in life and relationships head-on, resolving them, and moving forward."

Kutcher, whose letter was reportedly one of dozens submitted to the judge to ask for leniency, said he was aware his friend had been found guilty, but wanted his character reference to be taken into account in sentencing.

"I do not believe he is an ongoing harm to society and having his daughter raised without a present father would (be) a tertiary injustice in and of itself."

Kutcher's wife and fellow That '70s Show actor Mila Kunis also vouched for Masterson, telling the judge he had "innate goodness and genuine nature".

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"Throughout our time together, Danny has proven to be an amazing friend, confidant, and, above all, an outstanding older brother figure to me," she said.

"His caring nature and ability to offer guidance have been instrumental in my growth both personally and professionally."

Masterson was convicted in May on two counts of rape following a retrial. An initial trial on three counts deadlocked. Judge Charlaine Olmedo on Thursday ordered him to serve 15 years to life for each count, to run consecutively.

The three women at the heart of the charges against Masterson were members of the Church of Scientology at the time, as was he.