H-hahaha what secrets AREN'T being kept here? Both of these kiddos are sitting on a lifetime's worth of secrets, though they're not really deluding themselves about anything. ...well, Chara a little bit, but that's just their pessimistic nature.

So far as their natures, they really do read like two sides of a coin; Frisk is empathetic, kind, trusting, and Chara is prickly, wary, and reclusive. Both of them carry a lot of baggage and have a very low sense of self worth, but they have a very strong bond with each other and do their best to help prop the other up. Frisk in particular very clearly has a need to be loved, but also feels that they need to be useful and earn that love. Chara is...difficult. They do desperately want to be loved as well, but they've given up on ever having anything other than Frisk.