There is one soldier among Master Chief's legions that shows up pretty much everywhere he does - Chips Dubbo.
Chips Dubbo is a recurring NPC marine in Bungie's Halo series. Most well known for his Australian accent and his witty humour, Chips appears in Halo: Combat Evolved, Halo 2, Halo 3, and Halo 3: ODST.
Remarkably, he's one of the few marines to survive and fight alongside John-117 as he takes on the covenant, and makes repeat appearances in the games. Halo: The Master Chief Collection pays tribute to Chips in a number of achievements, usually obtained by 'listening to his words of wisdom.'
Chips, in many ways, is almost as lovable as Cortana, as he too accompanies Master Chief on many of his missions. He's voiced by Melburnian Andrew McKaige, who, funnily enough, had a brief role on Neighbours.
What a top bloke.
Bonus: check out this B-Roll of audio recording featuring everyone from Johnson to Cortana, with old mate Chips in there at 12.30: