In DRAGON BALL LEGENDS, you receive Z Power from Summons, not characters.

When you have acquired 100 Z Power for a character, you will unlock that character which you can then use in battle.
If you obtained the same Z Power of already unlocked character again from Summons, it will be added to the Z power you already posses.

You can confirm characters that you have acquired through the following:

■ Menu > Characters > Character List > Select “Filter” at the top of the screen, then check that all boxes have been checked and tap “OK” > Check the newly displayed Character List once again.