She appears to be a teenager, but a little older than the members of the Secret Scouts. Despite this, she seems to be easily frightened, and perhaps a little bit naive. This may be the result of a sheltered upbringing.
Her father was testing a perfume called "Beauty Mist", which attracted the notice of the Tyrannos. It unknown if Steffie was testing it for her father, or if she happened to be simply at the wrong place at the wrong time when the Tyrannos attacked Techodyne.
Bonehead was captivated by her beauty at first sight. She was terrified of him, but after Bonehead risked his life to save her from Brachio and Terrible Dactyl, she grew fond of him, even calling him "beautiful".