I just hit lvl 15 and saw I can access it. Of course not following my warlock ways I swapped over to it to do a level in the bad part of town (no bombs or glide ect).
If I stick with the primary will the subclass grow or do I need to do some lower missions to work them up. I had just hit venus and missed my primary very quickly with the vex coming up on me menacingly.
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Once you get your double jump and grenade and super it runs pretty smoothly after that. I started again after having hit level 15 because hitting level 15 unlocks all classes subclass on new characters. So feel free to recreate and be maxed at 20 on a subby.