In a Modern Toolchain
This example sets upright math letters, \symup and \symbfup, to Neo Euler. It leaves digits and operator names unchanged.
\documentclass[varwidth, preview]{standalone}
\usepackage{mathtools}
\usepackage{unicode-math}
\defaultfontfeatures{Scale=MatchLowercase}
\setmainfont{TeX Gyre Pagella}[
Scale=1.0,
Ligatures={Common, Discretionary, TeX}]
\setmathfont{Asana Math}
\setmathfont[range={up/{Latin,latin,Greek,greek},
bfup/{Latin,latin,Greek,greek},
cal, bfcal, frak, bffrak},
script-features={},
sscript-features={}
]{Neo Euler}
\newcommand\upi{\symup{i}}
\newcommand\upe{\symup{e}}
\begin{document}
\begin{align*}
\upe^{\upi x} &= \cos{x} + \upi \sin{x} \\
\upe^{\upi \uppi} + 1 &= 0
\end{align*}
\end{document}
This sets the Euler identities in Neo Euler, according to ISO style. Symbolic constants such as i, e and π are set upright, in Neo Euler, and everything else is from a Palatino clone. Variables are italic, and digits and operator names match the text font.
In Legacy NFSS
\documentclass[varwidth, preview]{standalone}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage[utf8]{inputenc} % The default since 2018
\usepackage{mathtools}
\usepackage{newpxtext, newpxmath}
% If you also want to load Euler Script and Fraktur, you want mathalfa.
\DeclareSymbolFont{eulerup}{U}{zeur}{m}{n}
\DeclareMathSymbol{\uppi}{\mathalpha}{eulerup}{"19}
\DeclareMathSymbol{\upi}{\mathalpha}{eulerup}{"69}
\DeclareMathSymbol{\upe}{\mathalpha}{eulerup}{"65}
\begin{document}
\begin{align*}
\upe^{\upi x} &= \cos{x} + \upi \sin{x} \\
\upe^{\upi \uppi} + 1 &= 0
\end{align*}
\end{document}
This version gives similar output in pdflatex, but does not define a separate upright math alphabet. Instead, it declares the individual upright constants it needs from eulervm. The encoding is documented in the amsfonts User Guide, but is the same as OML.
It is also possible to make the upright font (Euler) the default by loading the right package (eulervm or eulerpx), and then load a separate \mathit alphabet. However, the LaTeX kernel does not define an upright math alphabet separate from \mathrm, like \symup in unicode-math.
