Question: ```
\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\begin{NiceTabular}{>{\arabic{iRow}}cc>{\alph{iRow}}cc}[first-row]
    \multicolumn{1}{c}{}\Block{1-2}{Name} && \Block{1-2}{Country} & \\
    & George && France \\
    & John && Hellas \\
    & Paul && England \\
    & Nick && USA \\
\end{NiceTabular}
\end{document}

```

[![enter image description here](https://i.stack.imgur.com/AFvDj.png)](https://i.stack.imgur.com/AFvDj.png)

Questions

1. Is my code ok?
2. Is there a better code? (with nicematrix and without `\multicolumn` and first-row)

Answer: 
The second `NiceTabular` reproduces the first without using `\multicolumn` or `first-row`.

[![a](https://i.stack.imgur.com/oh4pj.jpg)](https://i.stack.imgur.com/oh4pj.jpg)

`\NRow` will output the row number decremented by 1 from the second row forward. `\ARow` does the same with alphabetical output.

```
\documentclass{article}
\usepackage{nicematrix} 

\begin{document}
\begin{NiceTabular}{>{\arabic{iRow}}cc>{\alph{iRow}}cc}[first-row]
    \multicolumn{1}{c}{}\Block{1-2}{Name}   && \Block{1-2}{Country} & \\
                                & George    && France \\
                                & John      && Hellas \\
                                & Paul      && England \\
                                & Nick      && USA \\
\end{NiceTabular}

\bigskip

% *************************************************** added <<<<<
\newcounter{iRowtmp}
\newcommand{\NRow}{\ifnum\value{iRow} >1 \the\numexpr\value{iRow}-1\fi}     
\newcommand{\ARow}{\ifnum\value{iRow} >1 \setcounter{iRowtmp}{\the\numexpr\value{iRow}-1} \alph{iRowtmp}\fi}        

\begin{NiceTabular}{>{\NRow}cc>{\ARow}cc}
\Block{1-2}{Name}   &       & \Block{1-2}{Country}  & \\
                    & George&                       & France \\
                    & John  &                       & Hellas \\
                    & Paul  &                       & England \\
                    & Nick  &                       & USA \\
\end{NiceTabular}

\end{document}

```

**Interesting bit** (after @Alan Munn comment) : the `\ARow` command can be further simplified by suppressing the row number check as in

```
\newcommand{\ARow}{\setcounter{iRowtmp}{\the\numexpr\value{iRow}-1}\space\alph{iRowtmp}}    

```

It will still work on the first row (where `\iRow-1=0`} because `\space\alph{<\iRow-1>}` will produce only the white space.
Simplifying Simon's answer, you can simply add a conditional to the first column row code:

```
\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\begin{NiceTabular}{>{\ifnum\value{iRow}=0\else\arabic{iRow}\fi}cc>{\alph{iRow}}cc}[first-row]
    & Name&& Country\\
    & George && France \\
    & John && Hellas \\
    & Paul && England \\
    & Nick && USA \\
\end{NiceTabular}
\end{document}

```

It's actually not clear to me why the conditional code is only required in the one of the column specifications (it doesn't matter whether it's in the first or second column specification; either way it will suppress the counter in that row for all columns).

[![output of code](https://i.stack.imgur.com/ftTZL.png)](https://i.stack.imgur.com/ftTZL.png)