-ka feminine, -ek masculine
More specifically, a softening consonant (in Czech typically č, š, n) precedes -ka, -ek. This softening consonant, in turn, is often preceded by i/í, which involves a change in the last vowel of the original name.
If the original name ends in a consonant, the consonant (if it is not tender enough

) often mutates into a softer one.
Honza - Honzíček
Pavel - Pavlíček
Petr - Petříček
Vašek - Vašíček
Tomáš - Tomášek
Jirka - Jiříček
Anna - Anička
Pavla - Pavlínka
Petra - Petruška
Jana - Janinka/Janička

Lucie - Lucinka
Helena - Helenka