 If you liked
Suisei no Gargantia Specials
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 ...then you might like
Shingeki no Kyojin: Kuinaki Sentaku
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"Gargantia on the Verdurous Planet Specials" / "Suisei no Gargantia Specials" and "Attack on Titan: No Regrets" / "Shingeki no Kyojin: Kuinaki Sentaku" are both prequels to action shounen series that expand more on the backstory of side characters.
For both of these series tie-ins, additional context is provided and mature issues are tackled.