The 22-year-old will make his Crusaders debut in the match against the Chiefs in Christchurch on Saturday night, five months after the District Court in Hamilton fined him $750 and discharged him without conviction for the assault in July.
The Fiji-born back had initially signed to play with Irish side Connacht last year, but the club tore up the contract in the wake of the court case.
Robertson said he had "never" had any concerns about Reece, who was called into the Crusaders' pre-season camp after being spurned by New Zealand's other four Super Rugby franchises.
"He's a quality young kid, he's gone through that process with the incident... and he's come down here," Robertson said.
"He's a great human - I don't think there's a guy who has worked harder in our group.
"(He's) extremely humble and I'm glad that we can play a part in his life and help him be the best he can."
The court heard that Reece had dragged his girlfriend to the ground during a drunken argument on a night out in Hamilton, causing her to suffer facial injuries and bruises to her knee and waist.
Crusaders assistant coach Brad Mooar said Reece deserved a second chance despite his "serious mistake".
"We can see the genuine remorse and his acceptance that he's done wrong and is working to fix it. And improve as a person.
"He's with his partner and he has great support from her," Mooar said.
The Crusaders are top of the New Zealand Conference and will look to maintain their perfect start to the season in Christchurch as they target a third straight Super Rugby title.