> shared umbrella
> lost in translation
Think of it like a flirting event, along the lines of the wall slam (壁ドン, "kabedon"). Sharing an umbrella is typically reserved for lovers or a close friend. There's probably a less awkward way to express MC's thoughts in English, e.g. "Wait, we're sharing an umbrella?", but it's difficult to convey to Westerners the significance of two people under the same umbrella without spelling it out in a TL note (though you'd hope other cues would clue people in that this is more teasing from Kubo).
The umbrella is a romantic symbol: in other manga (and I'm sure in real life), you'll see "love umbrellas" (names of two people under an umbrella) on chalkboards, bulletin boards, carved into trees, and so on. It's equivalent to putting the names of two people in a heart, something you see in Western countries. Hopefully this makes sense!