I remember creating him in choir, then refining him in maths lesson.

I kinda think of him as Undyne's lieutenant, and treats her with so much respect it almost equals to Papyrus'.
The outfit is too...ugh, though.

I'll make a form.

Name: Smiley

Age: Not known, but he acts about thirty.

Gender: Genderless, but prefers pronouns like 'he' and 'his'

Heart color: Dark grey

HP: 8139

Where you encounter him: When Undyne spots you in the first patch of grass, she sends Smiley to see if anyone was there.
Defeating method:
Pacifist:
RUN AWAY (Same as Undyne) and dodge the attempts at throwing throwing his head at you, or he'll attack you again. The fleeing scene will last until the short paragraphs describing the Underground's story begins.
Genocide:
FIGHT until his HP is low, in which he'll be called back by Undyne.
Convo:
U: SMILEY! GET BEHIND NE!
S: Undyne... No... I...have to protect the Underground... I have to protect YOU!
*exits fight scene*
U: It's no use pushing on like this! Get away from the human! *backs from the screen* we'll find them.
S: *hesitates, then leaves the screen*

After that scene, he appears after you've defeated Undyne.
Pacifist: As Undyne chases you, he will, too. If he lobs his head at you, you'll get attacked by him, instead. He'll leave with Undyne after you helped her. He'll appear when you visit Undyne afterwards, standing guard at her door.
Convo:
S: Papyrus? Well, hello, there! And...!
P: Yes! I brought the human! We're here to meet Undyne!
S: ... *goes up to the door and knocks on it* Undyne? *backs away from the door*
Normal Undyne convo starts, but with Smiley standing guard outside.
Interaction:
1st time: Dude. You'd better not hurt Undyne or I'll... I'll...
2nd time: Are you gonna go in or not? *repeats if asked again*

Genocide:
Before you land a hit on Undyne, he intervenes and parries the stroke.
There is no encounter with Monster kid (in this version with OCs) in this scene. You go straight to Undyne when you encounter her. She is not in her Undying form at that time.
After he parries the stroke, he will change into the second form I drew at the bottom of the page.
(Oh yea. I'll see if I can make the ST for him, as well.)
He will party your strokes until you see the words 'His breathing got heavy' as his description, as sweat drips from his head. Then, you can hit him once, where his sword will break. As a last attempt, he will snatch at you with his arms and throw his head, which will follow you wherever you go. Then, he will kneel down, then you can kill him.
Convo:
S: Ha...ha...
...
You may have got past me...
But you...
You'll never get past Undyne!

Attacks:
Pacifist: Stabbing his sword down into the screen, taking off his head and lobbing it at you, before it rebounds all over the screen and returns to him.
Genocide: Same attacks but faster. He will reach down with his arms, one after another, so you have to stay in the middle, then quickly move left and right to dodge the arms, as they will be moving as if they wanted to bring you to Smiley.

Backstory: He wasn't much. He was simply half of a monster soul, so he was weak and crumbling constantly. Undyne decided to help him, when she found him dragging his feet and leaving a trail of dust behind him on the street. She allowed him to fuse with one half of a human soul (I'll explain.) He grew into his new, more physical form (genocide form), and he could mold his body into less intimidating forms( pacifist form), and take off his head if needed.
After being helped by Undyne, Smiley because devoted to serving her, joining the Royal Guard, then appointed himself as her lieutenant after becoming good friends with her, whether she liked it or not. She laughed it off and agreed.

Human soul: A dead human fell into the Underground, with its soul already starting to fade. The monsters didn't have time to save it, therefore leaving them with one useless human soul, which they keeped for future use if needed.

Human soul color: Dark red.

Phew. Done. *faints into oblivion at having created that near-OP OC*