Twilight SirenG 100

First, you need to reach lv 33 to unlock the Summoning Altar in your technology tree (requires 3 Ancient Technology points to unlock and 100 Stone and 20 Paldium Fragments to build).

Then you need to collect 4 Bellanoir's Slab Fragments (you can find them in the chests at the end of caves/dungeons) and turn them into Bellanoir's Slab (can be done on a production line).

Once you have done this, you can summon Bellanoir at the Summoning Altar. Once you have summoned Bellanoir, the Summoning Altar will be destroyed.

The Summoning Altar can be built inside your Base, so you can use all of your Pals assigned to that Base in the battle.
The Bellanoir boss is level 30 and has 294K HP, so I would recommend that you get all your pets in your base to at least that level before summoning.

The Bellanoir boss can't be captured, but once you defeat it, you will receive an egg that will hatch a Bellanoir of your own.