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Contrary to popular belief, as revealed on a December 20, 2007 edition of Rare's Scribes, the decision to redesign Tiny into a teenager was not Rare's call, but Nintendo's, which is appropriate considering Nintendo has retained full ownership of Rare's Donkey Kong characters since the latter company were acquired by Microsoft in 2002, and thus now have full creative control on how they're presented and portrayed.
One detail that is worth pointing out, and may provide an explanation to why Nintendo had no qualms with redesigning a character who was meant to be Dixie's younger sibling into a teenager, is that the Japanese localization of Donkey Kong 64 reportedly makes zero mention of Tiny being related to Dixie, instead just being described by the manuals as just a "Charming girl Kong". It's also worth pointing out that every game that has featured Tiny Kong since 2002, such as Mario Super Sluggers, Super Smash Bros. Brawl, and Super Smash Bros. for Wii U has also made zero mention of Tiny's relationship with Dixie Kong, which adds credence to the theory that Nintendo opted to ignore and essentially retcon Rare's initial lore of the two being siblings.
Rare Scribes on Tiny's redesign:
https://web.archive.org/web/20071224053939/http://www.rareware.com/extras/scribes/20dec07/index.html
Japanese wiki article on Tiny (英語版においては【クランキーコング】の親戚かつディクシーコングの妹という設定がある。 = In the English version, she is a relative of Cranky Kong and sister of Dixie Kong.):
https://w.atwiki.jp/nitendo/pages/1679.html
Japanese DK64 website that just describes Tiny as "charming girl kong":
https://www.nintendo.co.jp/n01/n64/software/nus_p_ndoj/chara/index.html
Gathering of Tiny's profiles and stats on Mario Wiki, notice how nothing after the western DK64 material mentions her relationship to Dixie:
https://www.mariowiki.com/List_of_Tiny_Kong_profiles_and_statistics
https://web.archive.org/web/20071224053939/http://www.rareware.com/extras/scribes/20dec07/index.html
Japanese wiki article on Tiny (英語版においては【クランキーコング】の親戚かつディクシーコングの妹という設定がある。 = In the English version, she is a relative of Cranky Kong and sister of Dixie Kong.):
https://w.atwiki.jp/nitendo/pages/1679.html
Japanese DK64 website that just describes Tiny as "charming girl kong":
https://www.nintendo.co.jp/n01/n64/software/nus_p_ndoj/chara/index.html
Gathering of Tiny's profiles and stats on Mario Wiki, notice how nothing after the western DK64 material mentions her relationship to Dixie:
https://www.mariowiki.com/List_of_Tiny_Kong_profiles_and_statistics
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