What main stat should I upgrade on a Conjurer class?

[Mad Prince]

I don't see how or why Constituion is a main stat or important on this class, as you are summoning and you will be at the backlines most of the time? Any guide or idea please

[BlitzBob NoPants]

You are probably best off putting points into intelligence if you want to sling spells when not buffing your summons.

The constitution part is probably because your summon is your main weapon, so all you have to do is buff it and stay alive.

[Scott_Morgan]

just do whatever, summoner can be paired with almost everything

[Iry]

I recommend Wits.

You need Wits on one person to be your spotter, to choose some of the best Persuasion dialog choices, and it really helps your summoner when they can go first in combat. Or if you walk into an ambush, your Wits character can run away before they get hit!

Since Summoning is agnostic about stats, the Summoner is a great choice to be your Wits character.