When asked what would happen if Danoff was somehow incapacitated, Hogan said, "We have a tremendously deep bench of portfolio managers who may not be household names yet but probably will be."

The problem is unique to Fidelity: the next-largest stock fund run by a single manager, the T Rowe Price Growth Stock fund, that’s run by Joseph Fath, is at $44.1 billion under assets, while the third largest is another Fidelity product, the Fidelity Growth Company that’s run by Steven Wymer fund, is at $39.2 billion.

'Absent-Minded Professor’

Since he took over Contrafund in September of 1990, Danoff has posted an average annualized gain of 13.3 percent, the best performance of any large-cap growth fund over the same time, according to Lipper data. The benchmark S&P 500 posted a 10 percent annualized gain over the same time, while the average actively managed large cap fund returned an annualized 9.4 percent.

Someone who invested $10,000 in Contrafund in September 1990 would now have $213,719, more than double the $103,058 gain from investing in the S&P 500. The average large-cap fund, meanwhile, would have returned $95,586 from the same investment.

That performance is one reason why Fidelity emphasizes the chance to work with him to potential recruits, and why he leads a meeting each year with all of the firm's analysts that is said to be standing room only. Known to carry around a spiral notebook with handwritten notes on stocks, Danoff meets with hundreds of companies a year, and has easy access to management at companies who are keen to receive his endorsement.

Danoff, who joined Fidelity in 1986 as an analyst, has a degree in history from Harvard and an MBA from Wharton. He's known to walk around with a bulging file folder and messy hair, a marked contrast from some of the slickly-groomed fund managers who often show up on television.

"I love Will Danoff, he's like the absent-minded professor and he has encyclopedia knowledge of stocks," said David Caruso, managing director of Danvers, Massachusetts Coastal Capital Group who oversees approximately $800 million in client assets. "If he were to leave I would have to think very hard about leaving my assets there."

Fidelity named John Roth as co-manager of another fund that Danoff had run by himself, the $27 billion Fidelity Advisor New Insights fund, in 2013. The move was interpreted by many fund analysts as a sign that Roth will eventually become Danoff's successor at Contrafund.

Ford Motor Co. announced last month it would drop Contrafund from its fund lineup, pulling $900 million from the fund. The company attributed the decision as part of a plan to "best suit the needs of our employees," while retirement plan analysts said Ford has been removing funds run by stock pickers.