Went with the less informal / more phonetic-based spelling of "let's go" because it matched the relationship between いこっか and the proper form of the phrase (いこうか).

Also, pretty sure this is Tonari-chan (tawawa). The hairstyle is similar, she has similarly swept/parted bangs, and the boy looks similar to the one paired with Tonari-chan.

That boy gonna make every man there jealous.

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MetalEye88 said:

Of all the pairings in this series, this is still so far the most eyebrow raising.

really cause this pair is probably closer in age than Ai-Chan and Nii-San

micsolice said:

really cause this pair is probably closer in age than Ai-Chan and Nii-San

I'd say it's the other way around. Nii-San's most likely in early twenties which Ai-Chan isn't too many years away from. While this girl is at least 16 and the boy's probably not even close to 12 yet.