This answer has tips on how to do it on Gnome or Vim, but these don't work on KDE. This bug shows that KDE don't support the ISO notation with Ctrl+Shift plus the character's hex code. Is there any other way I can do this with a keyboard (that is, without copying and pasting)?
Memorising hexcodes is madness. Use the compose key instead. It lets you combine characters in a mnemonic way. This is a feature of X, not just KDE, thus works everywhere. Some examples:
Each key is typed sequentially without holding down. See the file
Since I do not have a Sun keyboard, I do not have a physical Compose key. I remap the useless Caps Lock key as logical Compose key. Change this in System Settings → Region/Language → Keyboard Layout (kxkb applet) → tab Advanced → section Compose key position, or run the command
Here is the KDE bug on the issue: https://bugs.kde.org/show_bug.cgi?id=103788
I addressed this issue in an article that deals with typing RTL text. Although in most common desktop environments Unicode symbols can be typed by holding the Alt key and pressing the numeric keypad plus sign then the Unicode value in hex, KDE users cannot use this method as KDE relegates responsibility for implementing this feature to Xorg, and Xorg relegates to Qt, and Qt relegates back to Xorg.