Lonely Castle in the Mirror sensitively explores themes of gender identity through the character of Subaru, a boy who faces bullying for his feminine expression. The narrative portrays his journey with empathy, framing external challenges while affirming his identity. Other characters offer unconditional acceptance, fostering a powerful message of belonging and self-worth.

Lonely Castle in the Mirror features Rion, a trans boy, as a central character. His identity is portrayed with dignity and respect, forming a significant part of his complex character arc. The narrative affirms his worth, depicting his struggles as external challenges like bullying, while emphasizing acceptance and understanding among his peers. The film's overall stance is supportive and validating of transsexual identity.

The narrative explores the challenges children face within their biological families, highlighting the limitations of parental understanding and support. It strongly emphasizes the formation of a 'chosen family' among peers as a crucial source of healing and mutual understanding.