Unity Song is dominant to Serendipity Stare. In order to prove this, I need to show that Brady's father Aaron is pure Serendipity Stare. I'll start by showing Brady's pedigree, and then will go on to prove Aaron to be pure. Bear with me, this proof is a bit long!
We'll begin with Giselle, here is her pedigree:
Giselle is the Serendipity Stare cat I bought from Eleanor. PL4 6 Boy is the starter hiding Serendipity Stare. This starter was bred to a Toyger Caramel showing Exotic Journey and produced Anora, who shows Exotic Journey and hides the starter's hidden eye. When Anora was backbred to the starter, she produced Roane, who shows Serendipity Stare and is pure, as the starter and Anora both had to have passed their hidden, which is Serendipity Stare. When backbred to Anora, they produced Giselle, who is pure Serendipity Stare, having gotten Anora's hidden as well as Roane's Serendipity Stare.
Giselle was bred to Podrick who shows Tapestry Allure and hides Dry Olive. Their kitten Tessie is Showing Dry Olive from Podrick and hiding Serendipity Stare from Giselle.
Tessie was bred to Tyler, who shows Exotic Journey and hides Soul Whisper. Their kitten Elsa shows Exotic Journey from Tyler, and hides Tessie's hidden Serendipity Stare.
Elsa was bred to Rhett, who shows Galaxy Star. We know Galaxy Star is recessive to Exotic Journey and that Elsa hides Serendipity Stare, therefore we know that their kitten Dinah hides the Serendipity Stare.
Here is where the pedigree splits a bit, but all cats are traceable back to Giselle.
Giselle was bred to Ed, who shows Pewter. Their kitten Junie shows Pewter and hides Serendipity Stare.
Junie was bred with Del, who shows Violet (which is recessive to Pewter), and has Cupcake, who shows Violet from Del and therefore hides her mother's hidden Serendipity Stare.
Cupcake was bred with Marcos, who shows Royal Sapphire and hides Galaxy Star. Their kitten, Gem, shows Galaxy Star (recessive to Violet) and so must hide Cupcake's hidden Serendipity Stare.
Gem was bred with Midas, who shows Soul Whisper. Their kitten Danny shows Soul Whisper and hides Gem's hidden Serendipity Stare.
Now back to Dinah. Danny is bred to Dinah, who was previously established to hide Serendipity Stare. Their kitten Seaborn shows Serendipity Stare and hides it as well (pure Serendipity Stare), as both parents passed their hiddens.
Seaborn, pure Serendipity Stare, is backbred to mom Dinah, hiding Serendipity Stare. Their kitten Elvis is showing Serendipity Stare, and therefore pure (Seaborn is pure, and Dinah passed her hidden).
Elvis is now paired with Martyna, showing Galaxy Star. Their daughter Madonna shows Galaxy Star from Martyna, and hides Serendipity Stare from Elvis.
Madonna is bred with Seaborn, previously proven to be pure Serendipity Stare. Their daughter Holly shows Serendipity Stare, with Madonna passing her hidden, and is therefore pure.
Madonna is again bred with Seaborn, producing a pure Serendipity Stare boy, Gryffin.
Gryffin is then backbred with mom Madonna, producing Kyle, also pure Serendipity Stare.
Kyle is bred with Holly, who has been proven to be pure Serendipity Stare. Their kitten Pekisko is also pure.
Pekisko is backbred with Kyle, producing Aaron, a pure Serendipity Stare.
Aaron is bred with Snowy, showing Unity Song. Their kitten, Brady, shows Unity Song. Since Aaron has been proven to be pure Serendipity Stare, there is no other possible eye that could be hidden under Brady's Unity Song; therefore Unity Song is dominant to Serendipity Stare.
Brady's pedigree again: