The add-on LeTBS_Summoning allows you to summon entities in battle.

Call an Entity

Entities are called during an action sequence, with the command summon: ID, cells.

Create a Summon

Look at the configuration file, inside the Lecode.S_TBS.Config.Summons module. Here's is the default summon template.

"identification": {
    active: true / false,
    turn_order: "after_caster" / "remake",
    visible_in_timeline: true / false,
    type: "user_playable" / "ai_playable",
    kind: "actor" / "enemy",
    id: BATTLER_ID,
    tied_to_caster: true / false,
    stats: {
        default: VALUE,
        mhp: VALUE,
        mmp: VALUE,
        ...
    }
},

The called entity can be active or inactive. An active entity is added to the battle flow and will play his turns.
If turn_order is "remake", the turn order will be calculated once again, to take into account the summoned entity's agility. Otherwise, the entity will be summoned in the timeline just after the caster's turn.
The summoned battler depends on the kind and the specified id.
If the summoned entity is tied to the caster, he'll die whenever the caster dies.

Stats

The summoned entity stats can be based on the caster's. Inside the stats option, you can setup which stat is based on the caster's same stat. The default sub-option is here for all non specified stats.
For instance:

stats: {
    default: "80%",
    mhp: "200%",
    mat: "+20%"
}

Means that all stats except MHP and MAT will be equal to 80% of the caster same stats. The MHP otherwise will be equal to 200% of the caster MHP. The + sign in the last line specifies that the caster will keep his base stat, but will get an extra value from the caster. Here, 20% of the caster's MAT is added to the summoned entity's base MAT.