Mariko's 5, she has a voice of a little girl and is quite high pitched.

Here's her personality from the Elfen Lied Wikipedia:

At first, Mariko appears to be the typical sort of Diclonius those who work at the Diclonius Research Institute expect them to be. Toward human beings, she was sadistic and didn't hesitate to kill anyone who came near her. This animosity extended toward Saito, who at that point endeared herself to her enough for Mariko to refer to her as "Mommy" before realizing Saito was not her mother. Only the threat of pain kept her in line, and rather than show malice toward her handlers; she expressed a childish excitement toward maiming the other diclonius she would be allowed to fight.

Her sadism, however, is only a portion of her being and is likely a front caused by the various traumas during her very short life. Beating Nana up and taking her limbs off is little more than a game to her, like a child "tearing the wings off an insect" in the words of Shirakawa. She only appears to realize the strength of her powers regarding how it can help her. That empowerment can take the form of escaping from her tormentors. It could be taking revenge on them (such as during her initial introduction). She could see it all as a game (fighting Nana), or as a way to protect herself or her family (fighting Lucy, beating up Nana out of jealousy in episode 13), yet can only take her so far. She's incredibly precocious as well, able to trick Isobe into giving her the cell phone code to postpone her bombs' detonation and killing him afterward in the manga.

Underneath the vicious nature she developed from being imprisoned in the research facility, Mariko is still, at heart, little more than a 5-6-year-old girl who wants nothing more than to be with her family again. This desire leads to her being selfish regarding what she wants and also makes her jealous of those who have what she does not. This envy shows itself best when she interacts with Nana. Mariko is initially envious of Nana having friends at all when she's a Diclonius like her, and this envy morphs into outright jealousy when she learns of Nana and Kurama's father-daughter relationship, finding it unfair since Kurama let his real daughter be locked away in the darkness. Naturally, she takes her aggression out on Nana and taunts Kurama in doing so as a way to punish him for abandoning her and favoring another girl with horns. In both manga and anime, though, Mariko does reconcile with her father before dying. In the manga, she asks Nana to take care of him before sacrificing herself in an attack on Lucy, and in the anime, both father and daughter die together after Kurama apologizes for abandoning her.