The words you need for ‘one and the other’ depend on what you talk about. See the examples below. You should be familiar with Japanese counters in advance.
People
One is a boy and the other is a girl.
一人は男の子で、もう一人は女の子です。
Animals
One is white and the other is black.
一ぴきは白くて、もう一ぴきは黒いです。
Objects
One is expensive and the other is cheap.
一つは高くて、もう一つは安いです。