Their first interaction was back in 2.0, as well with it being their first presumed meeting. Sparkle disguises as Robin, Sunday's sister. He immediately sees through her disguise after indulging in her playtime a bit, and Sparkle offers him that she'll play the role of 'Robin' as a stand-in. He rejects this.
Following 2.1, during Double Indemnity, Ratio mentions that The Family is still streaming the rehearsal of Robin's ceremony. It is later referenced again during Heaven is a Place on Earth, where Aventurine asks Sparkle if she was the stand-in for the public, to which she doesn't give an answer to.
In 2.2, we play as 'Sparkle' pretending to be 'Robin', cleaning up the mess Aventurine left. This pretty much confirms that Sparkle was indeed acting as Robin. During The Only Path to Tomorrow, Sunday and Robin meet Sparkle disguised as 'Robin'. Here, Sparkle teases Sunday, speaking about how she 'knows what he's planning' and giving Robin questions to mull over. Sunday yells at her to stop, and they leave. Here it is also mentioned that the Head of the Iris Family, Maeven Ellis, requested her to act as Robin. Whether for the entire time or just to clean up is unspecified, although.
In Sparkle's trailer Monodrama, the audience can see she is fighting against The Past, Present, and Eternal Show, enemies introduced during 2.2 during the final boss fight against Sunday.
The Lightcone featuring Sparkle It's Showtime is set in the Grand Penacony Theater, where once again, the final fight against Sunday took place. Both of these items came out during 2.0, whilst the fight was during 2.2.
Sparkle calls Sunday 'Chicken-wing boy' during all the encounters she's had with him.
Quotes[]
“Alright, alright~ Just putting it out there... If you're ever in a pinch, remember I've got your back. I mean, who could resist a guy rocking spikes on his wings?”
— Sparkle to Sunday in 'Whodunit'
“I don't care what you're thinking, chicken wing boy, but I'm pretty sure our lovely Robin won't be appearing on stage. After all, you're well aware of what a sorry state this Dreamscape is in under the banner of Harmony... Penacony, the Land of the Dreams... is this truly the paradise you desire?”
— Sparkle, 'The Only Path to Tomorrow'
“...Stop it!”
— Sunday, 'The Only Path to Tomorrow'
“What's the rush, chicken wing boy? Did I get to you?”
— Sparkle, 'The Only Path to Tomorrow'
Fanon[]
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The ship of Sparkday isn't too widespread but rare when compared to other notable Penacony ships. Due to their few interactions on-screen, there isn't much content featuring them, but only enemies. Sparkday has 60+ works on AO3.
The EN VA's of Sparkle, voiced by Lizzie Freeman, and Sunday, voiced by Griffin Puatu, both share the role of ENA in Power of Potluck, being the Sad ENA and Happy ENA respectively.
Sparkle was the first one to give a nickname to Sunday (Chicken-wing boy).