Here are some Demon Slayer OC name suggestions based on the provided character traits:

Female Names:

• Hana Shizuka: "Hana" (flower) suggests beauty and grace, while "Shizuka" (quiet) reflects her reserved nature. The contrast hints at a hidden depth.
• Ayame Kagerō: "Ayame" (iris flower) represents elegance and flexibility, while "Kagerō" (heat haze/fleeting shadow) alludes to her speed and the duality of her personality (happy exterior, depressed interior).
• Tsukiyo Himari: "Tsukiyo" (moon night) evokes a sense of quiet mystery, and "Himari" (sun's warmth) suggests her outward happiness. The combination reflects the internal conflict.
• Kasumi Ren: "Kasumi" (mist) represents her elusive and quiet nature, while "Ren" (lotus) symbolizes purity and grace, even amidst hardship.

Male Names:

• Ren Kaito: "Ren" (lotus) again suggests grace and resilience, while "Kaito" (sea person/clever thief) hints at a hidden agility and perhaps a mischievous side.
• Shinobu Hayate: "Shinobu" (忍) means patience and perseverance, fitting for someone who hides their depression. "Hayate" (疾風) means swift wind, reflecting his speed and grace.
• Hikaru Tsukikage: "Hikaru" (shining/brilliant) represents his outward happiness, while "Tsukikage" (moon shadow) suggests his quiet and introspective nature.
• Sora Arashi: "Sora" (sky) suggests freedom and vastness, while "Arashi" (storm) hints at the hidden intensity and potential for emotional outbursts.

Unisex Names:

• Kōhaku: Meaning "amber," this name suggests warmth and beauty, but also a hidden depth and complexity.
• Aoi: Meaning "blue," this name suggests calmness and serenity, but also a hint of sadness.

These names aim to capture the essence of a character who is outwardly happy and quiet but inwardly depressed, flexible, fast, and graceful. The choice between male, female, or unisex names depends on your preference. Consider which name best embodies the specific nuances of your character's personality and backstory.