I don't know. That's just always what I gathered. Unusually close, lived together prior to SS, their loyalty to Kenpachi filling the gap of a strong father figure. Not too mention Yumichika's general disposition and Ikkaku's overcompensation for something...perhaps batty tendencies. Seemed pretty unphased when dude's sister was obsessed with him right before the Hueco Mundo arc. Hmm. Just my theory.