Customizing the Sub-entries of the Index for a Key Person Entry

You can use something like

index{Person, Key@textsc{Person, Key}!NOOP{<number>}<description>}

where <number> is autogenerated and NOOP is a macro that does nothing, but is recorded in the .idx file. The number is padded to five digits with zeros, so it will be used for sorting.

Any “Person, Key” pair defines a counter that's incremented at each usage. Of course, we'll use a macro for automating the process.

documentclass[12pt]{book}
%usepackage{xparse} % not needed with LaTeX 2020-10-01 or later
usepackage{imakeidx}
makeindex[columns=1]

ExplSyntaxOn
NewDocumentCommand{NOOP}{m}{}

NewDocumentCommand{personevent}{mm}
 {
  mlchristians_personevent:nn { #1 } { #2 }
 }

cs_new_protected:Nn mlchristians_personevent:nn
 {
  int_if_exist:cF { g_mlchristians_personevent_#1_int }
   {
    int_new:c { g_mlchristians_personevent_#1_int }
   }
  int_gincr:c { g_mlchristians_personevent_#1_int }
  % emit the index entry
  index{ #1 @ textsc{#1}! NOOP { __mlchristians_pad:n { #1 } } #2 }
 }

cs_new:Nn __mlchristians_pad:n
 {
  prg_replicate:nn { 5 - tl_count:v { g_mlchristians_personevent_#1_int } } { 0 }
  int_use:c { g_mlchristians_personevent_#1_int }
 }
cs_generate_variant:Nn tl_count:n { v }

ExplSyntaxOff

begin{document}

Here are some words.personevent{Person, Key}{In the beginning textbf{EVENT 1}}

newpage abc newpage abc newpage abc

More words.personevent{Person, Key}{This is followed by textbf{EVENT 2}}
newpage abc newpage abc newpage abc newpage abc newpage abc

More words.personevent{Person, Key}{Later, we have textbf{EVENT 3}}

newpage abc newpage abc newpage abc newpage abc newpage abc newpage abc newpage abc

More words.personevent{Person, Key}{Continuing with textbf{EVENT 4}}

newpage abc

More words.personevent{Person, Key}{So follows textbf{EVENT 5}}

newpage abc

More words.personevent{Person, Key}{Later comes textbf{EVENT 6}}

newpage abc newpage abc newpage abc

More words.personevent{Person, Key}{Followed by textbf{EVENT 7}}

newpage abc newpage abc newpage abc newpage abc

More words.personevent{Person, Key}{Then comes textbf{EVENT 8}}

newpage abc newpage abc newpage abc newpage abc newpage abc newpage abc newpage abc

More words.personevent{Person, Key}{Shortly thereafter Event 8 comes textbf{EVENT 9}}

newpage abc 

More words.personevent{Person, Key}{The end: textbf{EVENT 10}}

printindex

end{document}

Please, note that textsc{text} is the correct syntax instead of {scshape{text}} and that bf has been deprecated for 25 years.

The .idx file is

indexentry{Person, Key@textsc  {Person, Key}!NOOP {00001}In the beginning textbf  {EVENT 1}}{1}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00002}This is followed by textbf  {EVENT 2}}{4}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00003}Later, we have textbf  {EVENT 3}}{9}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00004}Continuing with textbf  {EVENT 4}}{16}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00005}So follows textbf  {EVENT 5}}{17}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00006}Later comes textbf  {EVENT 6}}{18}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00007}Followed by textbf  {EVENT 7}}{21}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00008}Then comes textbf  {EVENT 8}}{25}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00009}Shortly thereafter Event 8 comes textbf  {EVENT 9}}{32}
indexentry{Person, Key@textsc  {Person, Key}!NOOP {00010}The end: textbf  {EVENT 10}}{33}

The .ind file is

begin{theindex}

  item textsc  {Person, Key}
    subitem NOOP {00001}In the beginning textbf  {EVENT 1}, 1
    subitem NOOP {00002}This is followed by textbf  {EVENT 2}, 4
    subitem NOOP {00003}Later, we have textbf  {EVENT 3}, 9
    subitem NOOP {00004}Continuing with textbf  {EVENT 4}, 16
    subitem NOOP {00005}So follows textbf  {EVENT 5}, 17
    subitem NOOP {00006}Later comes textbf  {EVENT 6}, 18
    subitem NOOP {00007}Followed by textbf  {EVENT 7}, 21
    subitem NOOP {00008}Then comes textbf  {EVENT 8}, 25
    subitem NOOP {00009}Shortly thereafter Event 8 comes textbf  {EVENT 9}, 
        32
    subitem NOOP {00010}The end: textbf  {EVENT 10}, 33

end{theindex}