Kaijudo: Rise of the Duel Masters?
The series follows the adventures of a young boy named Rai who possesses the rare ability to befriend and duel alongside fantastical creatures from a parallel dimension.
The main boy, a blonde girl, and another boy capture monsters.
Meet Ray (red hoodie boy), Gabe (blue shirt boy), and Allie (blonde-ish girl):
There was something about a 'veil' between their world and the monster world.
TVTropes mentions a veil:
These five shaman warriors took the near powerless bodies of the Creature Rulers and, putting them into a deep sleep, hid them deep underground. Henceforth, The Order of the Kaijudo Duel Masters began. Their charge was to make sure the Creature Rulers stayed in hibernation and also to guard the Veil, making sure nothing came out and keeping the earth and its people ignorant that such things as "monsters" ever existed.
Fast forward to the present day. The Veil is weakening and creatures are coming through. The current generation of Duel Masters are kept busy sending the wayward beasts back home and sealing the rifts they came through, all the while not aware of what this could possibly foretell…
A 'secret society' of sorts stopped the monsters from crossing, or used them to battle the people who wanted to use them for evil led by a wise old lady.
They are the titular Kaijudo Duel Masters, fighting against the Choten forces. The Choten are led by a formerly great Duel Master turned villain, want to use the creatures to rule the world, blah, blah, blah.
The old lady you remembered would be Master Jaha.
Another brown haired girl joined because she found out? There was an arc where the blonde got jealous and the monster she controlled attacked the brown haired girl?
Meet Lucy, a classmate who indeed came to discover the creatures' existence, and was allowed to tag along as an acolyte. Lucy is brown-haired.
Her and Ray don't seem to have a thing, but there's another girl (Alakshmi, a baddie who later joined the light side) who once kissed Ray and made Allie jealous.
In the end [...] the main boy and the blonde girl held hands... or kissed?
The ending of season 1 has Ray and Allie holding hands, which prompts Master Chavez and Nadia (two adult characters) to hold hands as well. Master Jaha (the old lady) is there too, and tells them to "kiss already", which they gladly do.
Found with the Google query
cartoon summon monsters "veil" which returned the TVTropes link above.