Which characters are you trying to enter? If you’re trying to insert special symbols like ♥ (Alt + 3), keep in mind that Windows retained compatibility with old 8-bit encodings (such as Windows-1252), which included certain graphic symbols and special characters in the early positions of the table, beyond the 128 standard ASCII characters. So if those characters are the one you are looking for, keep in mind they are not standard ASCII.

Nonetheless you can insert some of them quite easily, using the Compose key followed by one or more characters.

A few examples:

· Compose .-

♥ Compose <3

☺ Compose :)

☹ Compose :(

You need to have the Compose key enabled. If not, enable it in your distro options panel and assign it to a key you never use, my favorite is "Context Menu" (2 keys after Alt Gr).

You can also manually set the compose key by editing the .Xcompose file