na-adjective
linear; straight-line
Describes something that follows a straight line, such as a shape, movement, or arrangement.
この道路は直線的で運転しやすい。
This road is straight and easy to drive on.
彼のデザインは直線的な形が多い。
His designs often feature linear shapes.
na-adjective
straightforward; simple; direct
Used for a person's character, way of thinking, or approach that is direct and uncomplicated.
彼は直線的な考え方をする。
He has a straightforward way of thinking.
もっと直線的に話してください。
Please speak more directly.
率直 means 'frank' or 'honest' in speech, whereas 直線的 describes a straightforward, uncomplicated manner or thinking style.
Compound of 直線 (straight line) + 的 (-like, -ish). The figurative sense extends the idea of a straight line to describe directness in character or thought.