While Ni is electrodeposited on Ni electrode means reduction process happen at working electrode and then oxidation will happen at counter electrode. If your electrolyte made with DI water then water oxidation (Oxygen evolution reaction, OER) will happen at counter electrode.
I completely agree with our collegues, the main anodic reaction will be the oxygen evolution one, but if you applie high current densities with sulphate media you may also generate persulfate.