welcome, Rou.
I agree. 2. is correct. 1. is ungrammatical:
1. Rou seldom tell jokes.
2. Rou seldom tells jokes.
The reason 2. sounds odd to you may have to do with the position of the ver "tells". In English, the verb usually comes directly after the subject:
EX: Rou tells jokes.
The subject "Rou" agrees in number with the verb "tells" (-s on 3rd person singular verbs). Both the subject and the verb are singular. Here's a plural version:
EX: Rou and Casiopea tell jokes.
The subject "Rou and Casiopea" is plural and so it agrees in number with the plural verb "tell" (no -s on plural verbs).
Add in the adverb "seldom" and the verb "tells" looks out of place:
EX: Rou seldom tells jokes.
EX: Rou tells jokes.
Hope that helps.
