If you notice...
During the London singularity there's a lot of moments that Jekyll and Mordred show affection and care for each other. Just look at this moment, when they needed to move parts of the building's ruins (I don't wanna give spoilers by giving too much info) so they could go underground investigate and Jekyll offered to help Mordred even though he's not as strong as Mordred physically.
HE EVEN STAMMERED AND BLUSHED WHEN HE OFFERED HER HELP
And not too long ago (few chapters before) Mordred got worried about his safety when he decided to come along but Jekyll said he fully trusted his life to her. The whole London singularity it has them flirting and blushing around.
Imagine before our MC and Mash arrived to this singularity, how many moments may happened between them~ I personally like to imagine because before Gudako/Gudao & Mash arrived, only Jekyll and mordred were together in his house.