depend, rely on, father
怙 centers on leaning on someone or something for support: depending on or relying on another. The father meaning is a specific traditional application of that idea, referring to a child's reliance on a parent.
怙 combines the heart radical 忄 with 古, which likely contributes the sound. The heart suggests an emotional or mental leaning, fitting the idea of depending on someone.
The heart radical 忄 beside 古 (old) suggests an old, trusted person you lean on emotionally. Picture a child depending on an old father figure for support.
For コ, imagine a child calling out 'Co!' to an old father figure, relying on him for help. The sound 'Co' cues コ, and the heart radical shows the emotional dependence.