Dean McKenzie has many skills listed on his attribute board at present.
The basic one he started with was [Bone Fang].
He chose to enhance the skill of summoning in [Summon Skeleton].
There's the universal skill [Appraisal].
Then there's the [Meditation] skill used to restore mana, and the newly awakened, yet unpracticed [Slow Curse].
And also the once enhanced skill [Bone Shield].
This last one can be ignored since it had been enhanced, and it can be upgraded again when it reaches level 3.
The second one, [Summon Skeleton], can also be temporarily set aside.
After all, the skill has already been enhanced. Enhancing it further would only increase the strength of summoned skeletons or the number of skeletons summoned.
For the former, Dean McKenzie has the Gate of Bone, and for the latter, he has the talent to summon skeletons.
So, there's no real need to upgrade this skill at the moment, unless Dean has an ample quota for enhancements.