Trainer Heroes do not have a class and thus cannot be improved beyond their current state.
Trainers have stats that are generally somewhat better than unleveled heroes of the same star, but well below that of one that is fully maxed. They all have the same Special Skill, which is a lackluster 150% damage to a single target.
(Credit to Pois1 for the graphic)
Use in Battle
Generally speaking it is not recommended that anyone utilize trainer heroes in battle. The exception to this being if you acquire a 3* trainer hero when you have no leveled 3* heroes, yet. Because they come fully leveled and ascended they have the potential to do some decent damage compared to regular 2* heroes.
That said, trainer heroes have the weakest Special Skills in the game. This alone makes them a poor substitute for a real hero of equal level.
Use in Leveling Up
Because food costs increase with each new level of the hero that is leveling up, it is recommended that one saves trainers for the latter levels. Trainers provide so much experience that one may jump numerous levels at once. Since the cost for each hero sacrificed (with a maximum of 10) is equal to the cost of the first one selected, this allows one to skip the final and most expensive levels, thus conserving food when it is most expensive.
It is also generally recommended that they be used only in leveling 4* and 5* heroes.
Trainer Hero Experience Values
|Star Level||Basic 1/1 Hero||Trainer|