here today gone tomorrow (adjective) fly-by-night: unreliable and shady
Sinonime: assertive,
audacious,
bold,
brazen, certain, cocksure,
collected,
complacent,
composed, confident,
cool, gung ho, gutsy,
high*, imperturbable, overconfident, poised, positive, puffed up, pumped up, pushy, rosy,
sanguine,
secure, self-assured, self-confident, self-possessed, sure, unflappable, unhesitating,
upbeat*
Antonime: confused, dismayed, distrustful,
doubtful, doubting,
fearful,
hesitant,
nervous