unflappable (adj.)

• balanced
• bovine
• collected
• composed
• constant
• cool
• dispassionate
• dull
• easy
• even-tempered
• fast
• fiducial
• firm
• fixed
• flinty
• immovable
• impassive
• imperturbable
• inflexible
• loyal
• nonchalant
• patient
• predictable
• relaxed
• reliable
• secure
• set
• settled
• solid
• sound
• stable
• staunch
• steadfast
• steady
• steely
• stoic
• stolid
• substantial
• true
• unbending
• unflinching
• unruffled
• unshakable
• unshaken
• unswerving
• unwavering
• unyielding
• well-balanced