Lib Dem deputy Simon Hughes has said, despite being unable to vote for the government's tuition fees rise, he has "work to do" to explain it.
Mr Hughes was appointed as an advocate for access to higher education even though he abstained on the vote to raise the cap on fees in England.
He said the decision had been taken and now it was important to maximise youths' chances of further education.
But Labour's John Denham criticised the appointment as "window dressing".
Mr Hughes will be tasked with going into schools to address the concerns of poorer teenagers and work with them on campaigns to persuade as many as possible to consider higher education.
The job will last for six months and will be unpaid.
Announcing the appointment, David Cameron and Nick Clegg said it would be a "tragedy" if youngsters were put off applying for university due to "misinformation" about the government's policy.
They said details had been obscured by the political row over the controversial policy.
Earlier this month MPs voted to almost treble the cap on university tuition fees in England from £3,290 per year to a maximum of £9,000 - a policy which prompted large protests from students across the country.
Before the general election Lib Dem MPs pledged to vote against any increase in tuition fees.
However when it came to the vote, 28 Lib Dems supported the fee rises and eight abstained, or were absent from, the vote - making the party the focus of much of the protesters' anger.
'Independent of government'
Mr Hughes, one of those who abstained, will also contribute to work on finding a suitable replacement for the educational maintenance allowance (EMA).
Abolished by the Coalition government, EMAs offer low-income 16-19 year olds up to £30 a week to help them continue in full time education. Their abolition has also proved to be controversial.
Although deputy leader of the Lib Dems, Mr Hughes is not a minister in the coalition government and has previously raised concerns about various other policies.
He denied accepting that the appointment meant he would no longer be able to speak out on certain issues.
He told BBC Radio 4's World at One: "I'm independent of government. I'm not a member of the government, I've been asked to report to the government with ideas and recommendations."
The MP for Bermondsey and Old Southwark had previously considered voting against the policy, telling the BBC at the time he was concerned that the upper limit on the cap of £9,000 could put some youngsters off going.
But he told the World at One: "I'm very clear that the decision in Parliament has been made and the job of all of us across parties is to make sure we maximise the opportunity for young people of all backgrounds, particularly those who are not traditionally used to going on to post-16 education, to go to further education colleges and university."
He said: "I'm not there to sell [tuition fees]. I'm there to make sure people understand the facts and I know that is possible from my own experience in recent weeks and months in my own constituency... The problem with the system is the perception rather than the reality."
He said he still believed it would be better to have no tuition fees at all - but the realities of coalition politics meant the Lib Dems had to compromise on the policy.
He said there was "much more work to be done" - including making sure whatever succeeded the educational maintenance allowance was "as fair as possible" and that scholarships worked "to the maximum benefit".
"Above all, really importantly, that we don't have most universities opting for a fee higher than £6,000 a year unless there is absolutely clearly A, only that as an exception not the rule and B, very tight controls to make sure they deliver the sort of access that young people from my constituency would be able to benefit from to go to prestigious universities."
Lib Dem leader and Deputy PM Nick Clegg argued Mr Hughes was "ideally suited" to the role as a champion of young people from deprived backgrounds.
He said: "I know Simon will be tireless in seeking the best ways to communicate the opportunities open to young people, just as he will be a strong advocate for them to government."
But shadow business secretary John Denham told BBC Radio 4's Today programme Mr Hughes should apologise for not voting against tuition fee rises.
He said the government was "clearly worried" that students would be put off by higher tuition fees.
"Simon Hughes is as responsible as anyone in this Parliament for that. This is a terribly cynical piece of window-dressing."