Hichigo (Hollow Ichigo) was seen as an enemy for obvious reasons, he clearly stated on numerous occasions that his primary objective was to take over Ichigo's body. In a way Hichigo was an enemy to every main villain because of them trying to destroy Ichigo (his body) which resulted in an 'enemy of my enemy' complex. After Hichigo merged with Zangetsu though we begin to see a change in personality as a result of the combination of Zangetsu's love for Ichigo and Hichigo's dominant power. I'd say that only after this, is Hichigo really considered an ally. But that's my personal opinion.
Whether or not Hichigo will reemerge during this final arc though is possible but unlikely in my opinion as I'm quite sure it's been suggested recently that the members of the Vandenreich cannot steal Ichigo's bankai making him a very dangerous opponent for most of them. After considering this and keeping in mind any other new power-ups he may recieve, it's more likely for him to gain new power as it's a new arc but with that being said, it's also possible we may see Hichigo again. I personally would like to, he was a great character and I enjoyed watching him ravage as well as taunt his opponents which was basically the opposite of how Ichigo faught.