The Buddy system was introduced in the code base in 0.35.0 update.
Heres Niantic, explaining how itll generally work:
The next addition to Pokemon Go will be Buddy Pokemon.
Youll also have the ability to change your Buddy Pokemon at any time.
https://kotaku.com/pokemon-gos-buddy-system-will-let-you-have-special-time-1786107781
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source: www.techworm.net
