A problem with fonts and unicode is that many of the characters have no glyphs in a given font. This is going to bite us immediately unless we address it somehow. Our solution, inspired by Ergo Emacs, is to switch to a different font when rendering emoji. I've chosen to use Google Noto as my font for this, since it is dedicated to full Unicode coverage:

(defun mak::ghost-or-percent ()
  (interactive)                                                   ; Allow call via M-x or binding
  (let ((ghost "👻")                                              ; My desired prompt
        (percent "%"))                                            ; My fallback prompt
    (if (and (display-graphic-p)                                  ; Are we in the GUI?
             (member "Noto Emoji" (font-family-list)))            ; Do we have the emoji font?
        (progn (set-fontset-font t ?👻 "Noto Emoji" nil 'prepend) ; Use the emoji font for the ghost
               ghost)                                             ; Return desired prompt
      percent)))                                                  ; Return fallback prompt

I like my prompts simple. Prior to this, my prompt has been a lone percent sign for the last decade. I'm keeping the percent sign as a fallback in case I'm on a terminal, or a host missing the Noto Emoji font.

The set-fontset-font call is the heart of the above function, so let's look at its description:

(set-fontset-font NAME TARGET FONT-SPEC &optional FRAME ADD)

Modify fontset NAME to use FONT-SPEC for TARGET characters.

NAME is … t for the default fontset.

TARGET may be a single character to use FONT-SPEC for.

FONT-SPEC may one of these:
 * A font name string.

Optional 4th argument FRAME is a frame or nil for the selected frame that is concerned in the case that NAME is nil.

Optional 5th argument ADD, if non-nil, specifies how to add FONT-SPEC to the font specifications for TARGET previously set. If it is ‘prepend’, FONT-SPEC is prepended. If it is ‘append’, FONT-SPEC is appended. By default, FONT-SPEC overrides the previous settings.

The end result of this is that we've configured Emacs to always display the ghost emoji in the Noto Emoji font.

(defun mak::eshell-prompt ()
  (concat (if (not (eq 0 (current-column)))
          " "))

(setq eshell-prompt-function 'mak::eshell-prompt)

Eshell has special behaviour when you have the point after the prompt, honouring input as commands, so we need to explain to it how to recognize our newly-customized prompt.

(setq eshell-prompt-regexp (format "^%s " (mak::ghost-or-percent)))

Finally, we'll expand my war on welcome messages into Eshell:

(setq eshell-banner-message "")

From here I can run M-x eshell and have a functional shell inside Emacs.