Eichhorn, 16, joins the Bundesliga club on a long-term contract after Bayer Leverkusen beat off competition from Bayern Munich, Borussia Dortmund, RB Leipzig, Manchester City and Liverpool for his signature.
Rolfes described the approach that set Leverkusen apart in an interview with Bild.
"We've known the family for a while, which was a major asset in the conversations. They were already here last year, they followed Bayer 04 closely and built a good connection with various people across the youth and professional departments.
"We, as an entire club, campaigned and fought for Kennet. That made an impression. Kennet's parents said it was something very special."
The final confirmation came after a meeting with new head coach Carles Martínez, with Rolfes also present.
"I said to Kennet: you haven't played for us yet, but you already know half the club," Rolfes added.
The teenager leaves Hertha via a release clause reported at €8-9 million. He made nineteen appearances for the second-division Berlin club last season, becoming the youngest goalscorer in the 2. Bundesliga. Eichhorn will wear the number eighteen shirt.
In an unusual arrangement, the move will extend beyond the pitch. Eichhorn is required to complete a vocational placement year following his technical school certificate, and Rolfes confirmed this will be done within the club itself.
"Kennet still needs to complete a placement year after his vocational school leaving certificate — he will probably do that here at the club."
Hertha have secured a ten per cent sell-on clause from the transfer.