He joins from KPMG where he was a director and head of investment strategy.
Mr Willis will be responsible for the expanding firm’s asset allocation advice and manager research across all asset classes.
The company said he will also be “spearheading innovative and market leading investment research across the investment team.”
He has 16 years’ experience advising trustees and corporate sponsors of pension schemes on strategic investment matters.
At KPMG Mr Willis was responsible for overseeing all investment research across the UK team as well as being a client consultant to medium to large pensions schemes.
He was described as a “specialist in risk management techniques including liability driven investment and capital markets.”
Patrick McCoy, head of investment at Xafinity Punter Southall said: “The strength of any consulting business is only as good as the people it can recruit and retain.
“I worked with Simeon for 10 years and know just how good he is at developing thinking and advice that really adds value to clients.
“I am delighted he is joining the team we have put together following the merger of Xafinity and Punter Southall.”
Mr Willis said: “I am excited to be part of a growing business with big ambitions.
“In challenging markets, I want to help clients navigate all the investment opportunities to find the ones that will really help them.
“That’s what excites me.”