The problem with magic powers like creating short living black holes or rewire spacetime is that they can be generalized and used for many things. How do you justify that your super hero can only reflect damage with it?
Damage is a very abstract concept born from our intuition. It doesn't build up on physics.
Either go soft magic or bring something more physical that can explain most types of damages.
I propose to base this super power in kinetic energy or better, all types of energies. Kinetic energy is more a concept, a mechanic, than a thing that really exists. But it is consistent and well defined. The problem is preventing their possible generalization. If we want it to be able to reflect damage and nothing more.
With Kinetic Energy alone you cannot reflect things like burns, at least not directly. But you can inflict burns by friction. It can be used for more than just returning damage. You don't want this situation: the hero make the villains damage him a lot, then with all the stored energy, he demolishes a wall and escape.
Possible soft magic with hard mechanics solution: A god/witch/alien/five_dimensional_beings_from_interestellar gave Null the special skill to store all types of energy, but he can only use it to damage who damaged him first. If he uses it for anything else, he will be able to do that only once and immediately lose the special skill.
Why I thing this may work: a system that nobody understand, not even the one writing the story, is controlling the mechanics behind this power. It is expected that it won't even by explained (soft magic). But the mechanic of the power has very clear limitations and it's consistent (hard mechanics). The rules must also specify what is considered damage, and make this clear to Null.
Note: Null doesn't actually receives any damage, as the energies just feed the system. When I said "who damaged him first" I mean "who attempted to damage him". It is perceived as a reflection of damage, but it is more like store then release. The object storing the energies may be something outside the 3-dimensional space, to explain why It cannot be seen. This is a no go if you require instant reflection like when light is reflected, consider leaving this open to negotiation.