der Ukrainer
Ukrainian
A male person who comes from Ukraine or has Ukrainian citizenship.
Mein neuer Kollege ist Ukrainer und kommt aus Kiew.
My new colleague is Ukrainian and comes from Kyiv.

Ukrainian
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Ukrainian
A male person who comes from Ukraine or has Ukrainian citizenship.
Mein neuer Kollege ist Ukrainer und kommt aus Kiew.
My new colleague is Ukrainian and comes from Kyiv.

This noun is declined like an adjective. The ending changes depending on the article and case. You say 'der Ukrainer' (the Ukrainian, with the definite article), but 'ein Ukrainer' (a Ukrainian, with the indefinite article).
Der Ukrainer, den ich kenne, spricht sehr gut Deutsch.
The Ukrainian I know speaks German very well.
The word belongs to a word family: the country is called 'die Ukraine' (Ukraine), and the related adjective is 'ukrainisch' (Ukrainian).
Er ist Ukrainer und lebt seit fünf Jahren in Deutschland.
He is Ukrainian and has been living in Germany for five years.
The ending '-er' is often used to refer to male inhabitants of countries or cities, e.g. 'der Berliner' (the Berliner), 'der Amerikaner' (the American).
Ein junger Ukrainer hat den Wettbewerb gewonnen.
A young Ukrainian won the competition.
Russian
'der Russe' and 'der Ukrainer' both refer to a male person by origin or citizenship. But the difference is clear: a Russe comes from Russia, has Russian citizenship, or belongs to the Russian ethnic group. A Ukrainer comes from Ukraine or has Ukrainian citizenship. The words can be confused because both countries are in Eastern Europe and some people have multiple languages or cultural connections. Still, you should not simply swap the terms. Use the word that fits the person themselves.

Meine Schwester hat im Zug einen Ukrainer kennengelernt.
My sister met a Ukrainian man on the train.
Der Russe aus unserer Sprachgruppe erzählt gern von seiner Heimatstadt.
The Russian man from our language group likes talking about his hometown.